The Classroom Aquarium

Pets are a common fixture in many elementary classrooms all around the country. I still remember walking into my 2^nd grade teacher’s room and brimming with joy that she had a fish tank. And I’m not alone in that delight. Many researchers have found that classroom pets have a positive impact on students. Pets increase students’ positive attitude towards school and can actually make them want to come to school (Anderson & Olsen). They have been found to improve the emotional well-being of students and contribute to humane education (Daly & Suggs). Contrary to popular belief, one study showed pets did not distract from the teacher (Kortschal & Ortbauer).  And of all the possible choices for classroom pets, fish are the most common (Rud & Beck).

Freshwater aquarium (Neale Monks)

This is with good reason. Fish are relatively easy to care for. They don’t need to be fed daily, so they can easily be left over the weekend or a short break. During the summer, they can be taken home or just cared for on a weekly basis. There are also a wide variety of species that can be very easily acquired. It is also possible to keep fishes that are native to your location to give students a view of wildlife they don’t normally see.

Aquariums also have a well-established relaxing effect. Watching aquariums is known to reduce heart rate and blood pressure (Cracknell et al). And if I remember school correctly, tests certainly can generate some elevated heart rates! My mother is a middle school teacher and if there is one thing teachers need, it’s reduced stress levels. So in this way, the fish tank will help the teacher as much as the students. 

Lessons with an Aquarium

Aquariums can be very useful teaching tools. Students can examine how a fish tank attempts to reproduce a native habitat. How the light emulates the sun; how the filter gives the fish clean water like a stream; how the pump oxygenates the water like a waterfall. If native species are available students can observe their natural habitat and try to recreate it. Fish don’t have to be the only inhabitants in an aquarium. Plants can be added to further simulate a natural environment. Plants can also be used to introduce students to the idea of nutrient cycling.

Trout Unlimited volunteer helping with tank maintenance on the class's Trout in the Classroom project. Trout in the Classroom is a program started by the nonprofit Trout Unlimited which seeks to preserve freshwater streams and rivers around the USA. Students raise fish from eggs and release them at the end of the year. (Julia Ross, Troutfest.org)

Water chemistry and pollution are other lesson that can be taught with fish tank. Many pet stores carry small freshwater testing kits simple enough for middle school students. They can measure, record, and graph the temperature, pH, and nitrate levels in their fish tank. These water quality readings can be compared to local a river or pond. Teachers can actually let the fish tank get dirty to show students what happens to fish in polluted environments.

The nitrogen cycle is very important to success with an aquarium, and although it isn’t the same nitrogen cycle that happens in the air, it can be used to introduce students to the concept. Fish tanks can also help illustrate the water cycle from cloud to ocean. Students can even measure how much water evaporates from the tank on a weekly basis.

Math and science are very tightly linked, so an aquarium can also help with math lessons! Students can calculate the volume and surface area of the fish tank and convert those measurements from metric to standard units. From the volume, you can have them calculate the weight of the aquarium (hint hint, water weights 8.34 lb/gallon). These fun examples bring math out of the textbook and into real world.

Help Setting up a Tank

Right about now, you may be thinking, this sounds great but where do I start? Thankfully fish tanks aren’t difficult to set up and maintain, but they will require a bit of effort on your part initially. The first thing you will want to read and research is the freshwater nitrogen cycle. This is the article I wrote on it. Many many resources exist online to help you in starting a fish tank, and I encourage you to check out the other fish blogs I recommend for additional resources. My friend Mari at Aquariadise has some wonderful articles to help new fishkeepers such as types of filters & choosinga substrate. The National Science Teachers Association also has guidelines for keeping live animals in the classroom which I highly recommend reading.

Suggestions to a New Fishkeeper

If native fish are at all an option, I highly recommend them. In the eastern USA we have many stunningly beautiful fish right in our own backyard. You can read more about them in my article: The Beauty in Your Backyard.

 If native fish aren’t an option, then the next best suggestions I can make are small tetra like the glowlight (Hemigrammus erythrozonus) and X-ray tetra (Pristella maxillaris). Small barbs like cherry barbs (Puntius titteya) and gold barbs (Barbodes semifasciolatus) also make great classroom fish. Zebra danio/glo-fish (Danio rerio) are a staple and great, hardy fish. All of these species need to be in groups larger than 10 because they are shoaling species, meaning they survive best when in groups. They all need a tank larger than a 10 gallon, too. You can read more about these species in my article: Best and Worst Beginner Fish.

A few of the glowlight tetra in my personal tank. They are rather hardy and don't mind me leaving them for a weekend. 

For tank size, I recommend starting with a 20 or 30 gallon tank. It seems counter-intuitive but small tanks are actually harder to keep. Larger volumes of water mean there is more of a buffer in case something goes wrong. Larger tanks also mean you’ll be able to keep more fish which are always delightful to students.  

I highly recommend plants in an aquarium. They create a soothing natural environment and open new doors for educational opportunities in addition to helping keep it clean. To successfully grow plants you will need a plant-specific light. An aquarium kit should come with a hood and light, but that light isn’t going to be tailored to plants. You will need to get a 6500k full spectrum daylight bulb. Many pet stores also sell these, but hardware stores will have them if your pet store doesn’t. They only need to be replaced once a year to keep your plants growing green and beautiful. Some plant species that are a good, hardy beginner species are anubias, java fern, amazon sword, and duckweed. Pet stores often have these common plants for sale.

Help with Funding

Aquariums (like all pets) require money, but fortunately for you, there is a nonprofit organization that gives grants for teachers interested in getting a classroom pet. Pets in the Classroom allows teachers to submit proposals for grants to assist with setting up and maintaining classroom pets. Their website also contains lesson plan and habitat ideas for many species of pets. I highly encourage you to check it out. If regulations and demographics allow, you can ask for a small donation from your students to help fund your classroom pet experience.

Works Referenced

Anderson, K.L. and M.R. Olson. 2006. The value of a dog in a classroom of children with severe emotional disorders. Anthrozoös 19(1): 35-49.

Aquatic WILD: K-12 Curriculum & Activity Guide. 2013. Revised ed. Council for Environmental Education, Houston.

Cracknell D., M. P. White, S. Pahl, et al. 2015. Marine Biota and Psychological Well-Being: A Preliminary Examination of Dose-Response Effects in an Aquarium Setting. Environment and Behavior. DOI: 10.1177/0013916515597512

Daly, B. and S. Suggs. 2010. Teachers’ experiences with humane education and animals in the elementary classroom: implications for empathy development. Journal of Moral Education 39(1): 101-112.

Kortschal, K. and B. Ortbauer. 2003. Behavioral effects of the presence of a dog in a classroom. Anthrozoös 16 (2): 147-159.

“Responsible Use of Live Animals and Dissection in the Science Classroom.” National Teachers Science Association. www.nsta.org/. Updated: March 2008. Retrieved: 24 Nov 2015.

Rotman, E. 2008. Hatching Stewardship. Outdoor California (March-April): 33-35.

Rud, A.G. and A.M. Beck. 2003. Companion animals in Indiana elementary schools. Anthrozoos 16 (3):241-251.

Rutherford, B. 28 March 2015. Tanks in Classrooms: Setting Up an Educational Aquarium. Reef to Rainforest Media. Retrieved: 24 Nov 2015.

“Trout in the Classroom.” Trout Unlimited. www.troutintheclassroom.org/. Updated: Nov 2015. Accessed: 24 Nov 2015.

Freshwater Aquarium Nitrogen Cycle

Welcome to the most important article I or anyone can ever write about fishkeeping: the aquarium nitrogen cycle. Contrary to what you may hear at a pet store, it is not just filling your aquarium and running the filter for a week.

What Is It?

The nitrogen cycle is how we maintain a healthy environment, and it surprisingly relies completely on bacteria. These microorganisms change the toxic waste of our fish into a relatively harmless substance, thereby allowing us to keep fish in contained areas. In the wild, the fish waste is diluted by the humongous volumes of water. 

The cycle begins with fish. They produce ammonia as a byproduct of their metabolism. The problem is ammonia is toxic to them (and us for that matter). Luckily a genus of bacteria called Nitrosomonas exists. These bacteria take ammonia and convert it into nitrite to generate energy for themselves (much like plants use photosynthesis to generate energy from the sun). However, nitrite is still toxic to fish. That’s where second genera of bacteria called Nitrobacter and Nitrospira come in. They convert the toxic nitrite into nitrate which takes much higher levels to reach toxicity. However, nitrate is still toxic to fish and will need to be removed from the aquarium by its caretaker. This is one reason we perform periodic water changes on our aquaria.

Aquarium nitrogen cycle schematic. (public domain)

Nitrobacter and Nitrosomonas also have requirements just like our fish. They live on surfaces in your aquarium although a small amount can be found floating in the water. They operate best between the pH’s of 6.0 and 9.0 and need water above 50F. They also require highly oxygenated water, so they are most commonly found in the filter. For this reason, it is important to never wash your filter media in chlorinated water. 

When plants are added to an aquarium, they use up nitrogen in all three of the forms (ammonia, nitrite, and nitrate). If enough fast-growing plants are present, there may be very little ammonia that makes it to the filter. However, even with a planted aquarium, I recommend having some type of filter where these important bacteria can grow. Plants can be finicky and stop growing for odd reasons, and when that happens an ammonia spike can be deadly to fish and invertebrates. Having plants does not negate the necessity of the nitrogen cycle in your freshwater aquarium.

How to Establish a Cycle

Cycling an aquarium takes weeks to a month because the colonies of bacteria replicate slowly. Nitrosomonas need a source of ammonia to produce nitrite before Nitrobacter can begin to grow.  The only way to know when it is complete and what stage you are in is to test for ammonia, nitrite, and nitrate. First you will see an ammonia spike, followed by a nitrite spike, and then nitrates. You should test your water daily and record your results to chart your progress. A few different methods of getting ammonia into your aquarium and starting the cycle are detailed below.

Chart of expected ammonia and nitrite spikes while recycling a tank. (theaquariumwiki.com)

Live Fish

This is the most common and oldest method. Simply throw the fish in and hope for the best. The bacterial colonies will build up over time, but while you are waiting your fish will be subject to ammonia poisoning and nitrite poisoning in the weeks it takes the Nitrosomonas and Nitrobacter colonies to establish themselves. This is how many ignorant aquarists (including myself) originally establish a cycle, however it is not the most humane. Other alternatives should be explored first. 

Fish Food

In addition to living fish producing ammonia, decaying food will also produce ammonia and can be used to start a cycle. You can use fish food or raw seafood. However, this method isn’t without risks as rotting food will attract other bacteria and fungi. If you are using this method, you need to do a large (70% or more) water change before you add fish and invertebrates. 

Pure Ammonia

You can find this sold as a cleaning agent in many stores. Just make sure you check the label because the ones with surfactants will not work. Add the ammonia to your tank water until you get a concentration of 3ppm (any higher and it will inhibit the Nitrosomonas). When the ammonia concentrations drops to zero and you see nitrites, add more again. Keep doing this until adding ammonia produces nitrates. You will have to keep dosing the tank until you add fish to sustain the colonies. Before you add fish, do a water change to reduce the nitrate levels to below 40ppm. 

Seeded Filter

If you have a friend or store you trust, you can ask for some of their filter media to add to your filter. Because these bacterial colonies are most commonly found attached to filter media, you will instantly establish a cycle. You can add fish the day you add in the media. If you have multiple tanks you can borrow media from one tank to cycle another. Since I cycled my first aquarium, I have used this method exclusively to cycle my tanks. 

Bacteria in a Bottle

Nitrosomonas and Nitrobacter can be bought from pet stores and online. A few select products are believed to work quite well. Dr. Tim’s One and Only and Tetra Safestart are two that are claimed to work wonderfully. By adding these products you can cycle your tank instantly and add fish that day. You can also use these products in conjunction with any other method to increase your chance of success. I have not personally used these products at the time of writing, however I have heard amazing reviews from others. I do have plans to try these products myself one day.

Cycling an aquarium is the single most important thing you can do for the health of your fish. It is time and effort, but it is worth every minute for the headaches it will save you. The best medicine is clean water, and that is just what a cycle gives you.

Works Referenced

“Ammonia & the Nitrogen Cycle: Important Steps for YourAquarium.” Drs Foster and Smith. www.drsfostersmith.com. Updated: 18 October 2015. Retrieved: 18 October 2015.

“Aquarium Nitrogen Cycle.” Fishlore. www.fishlore.com. Updated: 30 September 2015. Retrieved: 18 October 2015. 

DeLong, D.P. and T.M. Losordo. 2012. "How to Start a Biofilter." Southern Regional Aquaculture Center. Retrieved: 18 October 2015. 

“Fishless Cycling.” Algone. www.algone.com. Updated: 15 March 2012. Retrieved: 18 October 2015. 

“’Fishless’ Cycling.” The Skeptical Aquarist. www.skepticalaquarist.com. Updated: 21 March 2011. Retrieved: 18 October 2015. 

Helm, Ben. 2014. “Controlling Ammonia in a Fish Aquarium.” www.fishchannel.com. Retrieved: 18 October 2015.

 Losordo, T.M., M.P. Masser, and J. Rakocy. 1998. "Recirculating Aquaculture Tank Production Systems: an overview of critical considerations." Southern Regional Aquaculture Center. Retrieved: 18 October 2015. 

Monks, Neale. 2011. “FishlessAquarium Cycling Method.” www.fishchannel.com. Retrieved18 October 2015. 

“Nitrifying BacteriaMixtures Work.” Dr. Tim’s  Aquatics. www.drtimsaquatics.com. Updated: 8 September 2015. Retrieved 18 October 2015.

Common Beginner Aquarist Mistakes

Okay, so you've gotten your first tank, and it’s fully cycled. Whew. Glad that ordeal is over. But you have to be careful. There are still a few beginner mistakes that will trip up a new fishkeeper. I’ll recount some of the big ones here and ways to avoid them.

Trusting the pet store implicitly

When you are just beginning you get a lot of advice from a lot of people. Some of it will be good, but some of it will be bad. The majority advice will from pet stores will fall under the bad category. The large chain stores like Petsmart and Petco are notorious for this. I can’t tell you how many times I’ve heard someone say “But this is what the lady at Petsmart told me to do,” when I asked why they were doing something wrong.

It’s actually in pet store’s short-term benefit to give bad advice because it leads to stress on the fish. Stressed fish get sick. And where do you buy medications to cure the illnesses? The pet store. So instead of just making money from the tank, the fish, the décor, and the filter, they also make money on the medications. And they sell a lot of them.

So to be safe, double check whatever advice you get from pet stores. For the most part they are thinking about money not your fish.

German blue ram (Mikrogeophagus ramirezi),
beautiful but fragile  (sapienssoultions @ Flickr)

Leaving the light on too long

While this usually won’t kill your fish, it will do your tank more harm than good in the long-run. I’ve heard people say they want to use the tank as a night light, but people often forget that fish need sleep, too. They don’t have eyelids and can’t block out light like we can.

All living thing have a circadian rhythm; this is basically your body’s clock. Scientifically it’s a series of hormonal responses that govern a variety of processes in the body from hunger to the need to use the bathroom. It makes you sleepy before you normally go to bed. It’s how your dog always knows when it’s feeding time. Fish have this, too. Part of the circadian rhythm is the day/night cycle. The circadian rhythms of fish are directly related to the amount of light. These changes in light govern things like metabolism, sleep, and breeding behavior. Sleep in fish is induced by an absence of light much in the same way as with humans. Without a period of darkness the hormones that govern these processes won’t be produced. As a result of this, the fish won’t live a normal existence.

Excess light in the aquarium will also lead to outbreaks of algae. Some algae is normal. No healthy tank is without it. But when it starts to take over the walls and coat the décor, it becomes a nuisance. One of the easiest ways to get rid of algae is to reduce the amount of light coming into an aquarium which usually means reducing the amount of time the tank light is on. This leads me to my next point.

Getting a fish to “clean the tank”

There is no fish that will do this for you. Algae eaters (and not all fish that are advertised as such will eat algae) may physically remove the algae, but it’s not gone. They just convert it into poop. This poop decomposes to become ammonia which is algae fuel. Combine this with the lights being left on too long and you have a recipe for even more algae.

I’ve also seen people get substrate fish like cories and loaches for the purpose of cleaning the tank. They want something to eat the food the other fish miss. But when these fish aren’t given some kind of supplement they often die of malnutrition (yes it is possible to underfeed). All fish in all levels of the aquarium must be fed. Even the true algae eaters like the oto catfish and farlowella catfish need supplements from time to time.

I’ll take this moment as aside to talk about the fish that are commonly sold as algae eaters and the problems most come with. These fish include plecos, Siamese algae eater, Chinese algae eaters, and oto catfish. These fish are often “buyer beware” as the labels in the pet store rarely tell you everything you need to know about them. Plecos only graze algae when they are juveniles and actually become predatory scavengers as they age. The adults need levels of protein similar to most community fish. They often achieve this by latching on to the sides of slower moving fish like discus and goldfish. Some species like the bristlenose (Ancistrus spp) will stay small enough for a regular community, but the fish commonly traded as “common pleco” will get over a foot long and need a very large aquarium. 

Young male bristlenose pleco (public domain)

The Siamese Algae Eater (SAE) is another one that can cause problems as there are three fish traded under this name: true SAE (Crossocheilus langei), false SAE (Garra cambodgiensis), and flying fox (Epalzeorhynchos kalopterus). All of these fish look remarkably similar, but have different behaviors. The true SAE and false SAE are both peaceful provided they are maintained in groups of more than 5 individuals, but false SAE won’t eat algae like the true SAE. The flying fox is the bad apple; they are rather nasty to members of their own species and any other fish that likes to be near the substrate. They hardly ever touch algae. The name of the Chinese algae eater (Gyrinocheilus aymonieri) is rather misleading as it won’t eat any algae either. It’s also rather aggressive and not recommended for either a community aquarium or algae control. Oto catfish are actually very good at keeping an aquarium free of algae, but they are a rather fragile fish and still need supplements to be healthy. 

Overfeeding

It’s another major problem that plagues the new aquarist. Every single time you walk by the aquarium, the fish are at the front, and they’re begging for food. They always look so hungry, and you want to feed them. Just because fish look hungry doesn’t mean you should feed them. It’s a good thing that your fish are acting hungry. A hungry fish is a healthy fish. But overfeeding can change that.

When you overfeed (add more than the fish can consume in a minute) then the extra food sits at the bottom and decays. This releases more ammonia into the water and contributes to the overall nitrate level in the tank. The decaying food will also lower the pH in the tank as well as use up oxygen. Even if you remove the uneaten food, the fish will poop more when they are fed more. This poop will also decay and cause the same problems as uneaten food.

Koi begging for food (Barbara L. Hanson @ Flickr)

“But they always look so hungry.” I know. But fish metabolisms operate much differently than a mammal’s metabolism. Because we have to spend so much energy maintaining our core body temperature we need to eat a lot of food. Fish don’t need half the calories we do because they don’t heat their bodies. In fact, most healthy adult fish can go about 2 weeks without eating and still remain fine.

Messing with water chemistry

I’ve seen a few beginners make this mistake, but not as many as you would think. Sometimes the pet store employees encourage the purchase of products to stabilize pH, and other times people just see them and assume they are necessary because they believe a perfectly neutral pH is essential for fish. In most cases these products are not needed at all and can actually harm the fish by creating a constantly changing environment as they rarely ever stabilize the pH. Fish are adaptable, and many of the best beginner fish will adapt to any pH within a normal range (6.0 – 8.0). Stable water is always safer than water with a rapidly changing pH.

Adding too many fish at once

This is probably the second most common mistake after trusting the pet store. Too many fish added to a tank too fast will cause problems in any tank, even the established tanks of old pros. Adding a lot of fish at once creates a large influx of ammonia. The beneficial bacteria that comprise a cycle don’t replicate that fast and need time to adjust to large changes. In the meantime, the ammonia is poisoning your fish. Whenever you add fish, you will have an increase in ammonia, but if you add them a few at a time, the influx will be small enough that the plants and bacterial colonies can easily take care of it and grow accordingly.

In an uncycled tank, adding a large amount of fish can be deadly. They will be producing ammonia at a fast rate and there will be basically no bacteria to take care of it. Ammonia levels can rise to fatal amounts within a matter of days. This is called new-tank syndrome. It’s easily avoidable by either cycling the tank with a small number of very hardy fish or by cycling the tank before you add the fish.

Harlequin rasbora shoal (Chantal Wagner @ flickr)

The most important thing to remember is this hobby is about patience. I've heard it said that nothing good happens fast. Plants and fish don’t grow overnight and neither do the good bacteria. If you need a “fish fix” in the meantime, join a fish forum and talk to other hobbyists!

Works Referenced

“10 Biggest Mistakes in Fishkeeping.” practicalfishkeeping.co.uk. Updated: 2 December 2011. Retrieved 1 Aug 2012.

Helfman, G. S et al. 2010. The diversity of fishes, second edition. Wiley-Blackwell, Oxford.

Zhdanova, I. V. and S. G. Reebs. 2006. Circadian rhythms in fish. Pages 197 - 238 in K. A. Slowman, editor. Behaviour and Physiology of Fish, vol 24. Gulf Professional Publishing. 

How to stock a fish tank

A very common beginner question is “how many fish can I put in my tank?” The process of adding fish to a tank is called stocking and is more of an art than a bright line rule. However, there are some generic guidelines that will help you understand the thought process that goes into choosing fish for an aquarium. When you are in doubt don’t hesitate to ask a more experienced aquarist.

Tank size

This is probably the single most important aspect. A jack dempsey (Rocio octofasciata) could hardly turn around in a 10 gal tank. An adult koi (Cyprinus carpio) would be bouncing off the walls in a 20 gal. Some fish simply won’t fit in certain tanks. Size matters.

Length

The most important subset of size is length. Look at the body of a typical fish. It’s streamlined for moving forward not up and down. Fish need room to swim side to side more than they need room to swim vertically.

Footprint

This is the how much surface area the water will have. It is dependent on the length and width of the aquarium. Since oxygen exchange happens through the surface of the water, the larger the footprint, the more fish you will be able to have. Conversely, tanks that are taller than they are wide will have a smaller footprint than a long tank of equal volume. This will limit the number of fish the tank can hold.

Volume

While this isn’t as important as length and footprint, it still had great impact on stocking capacity. The more of it, the better; it dilutes more waste and gives fish more room to swim. A good way to think about it is to look at a 20 gal long which is 30" L x 12" W x 12" H versus a 29 gal tank which is 30" L x 12" W x 18" H. Both of these tanks are the same length and width, but the 29 gal is 6 inches higher than the 20 gal long. They both have the same footprint and swimming space, but you will be able to add more fish to the 29 gal simply because it has a larger volume of water.

Pulling these all together

What all of this means is that you will always be able to have more fish in a long tank rather than a tall tank. Long tanks give fish more room to swim, have more water surface area for oxygen exchange, and will allow you to have more fish.

School of angelfish Pterophyllum spp (Mr. Sepia @ flickr)

Bioload

This is most easily defined as the amount of waste an organism produces. Waste can come in the form of solid or liquid. The total bioload of the aquarium (and therefore the total number and species of fish) is limited by the surface area the bacteria can colonize. The good bacteria in an aquarium live on solid surfaces instead of the water column. Without enough surface area to grow a large enough colony of bacteria to handle the bioload, the water will become toxic to the fish.

Not only does the shape of your tank matter, but the shape of the fish you plan to add matters too. While deep-bodied fish such as many cichlids, fancy goldfish, and puffers may be as long as some tetra and gourami, the deep-bodied fish will produce more waste because they have a greater total volume. You will always be able to keep more fish with a naturally slim profile in a tank than fish that are naturally thick. Larger fish have a larger bioload, so you will always be able to keep more small fish in a tank than larger fish.

While I’m talking about fish size and shape, I need to mention that you always look at the adult size instead of the current size. Yes a baby oscar could physically fit in a 20 gallon tank, but you will end up buying a completely different setup as it grows. Save your money and only buy one tank: the tank where it can live its whole life healthily. It’s not a good idea to buy a fish and plan to upgrade later because many things can happen in between now and when the fish needs the upgrade such as unemployment, family emergencies, personal injury, and any number of things that can remove your financial resources.

Poop matters

Oto catfish (Otocinclus spp) with
von rio tetra (Hyphessobrycon flammeus)
in the background (lejoe @ flickr)

The amount of fecal matter a fish produces is also important. Fish that eat continuously like many oto catfish, plecos, and goldfish produce a lot of poop. The poop breaks down into ammonia which is toxic to fish. The ammonia is made non-toxic by colonies of bacteria, but these bacteria need a surface on which to grow. In a small tank, the surface area is greatly limited, and the bacteria won’t have as much room to grow; this is one of the major factors that limit how many fish you can stock to a tank.

Water changes

While I’m talking about the biology of stocking, I should mention how water changes play a role in this process. While they won’t do anything for the physical size and swimming needs of each fish, they do affect how many fish you can add. A typical healthy water change is between 25% and 50% of the total tank volume once a week. The more water you change on a weekly basis, the more fish you can have in the tank. This is because water changes remove nitrates, dissolved organic matter like hormones, and other crud from the tank. These can’t be removed any other way. Without weekly water changes the continued presence of these molecules in high concentrations in tanks causes stress to the fish. Larger water changes won’t allow you to cram your tank full of fish, but it will allow you to add another 4 or 5 fish to your tetra or corydora school.

Compatibility

Not all fish can get along. Some will gladly eat anything small enough to fit in their mouths, and some are so aggressive they will bully anything in the tank. Before you put more than one species in a tank together, you need to check compatibility. There are some general trends you can follow, but always double-check the individual species before you buy them.

Gourami males are territorial with other gourami males. Most males can be housed with two or more females, but to house more than one male together, you will need a rather large tank as all males need room to establish their territories. The exception to this rule is betta fish. The males and females should never be housed together except for breeding purposes. 

Tetra, rasbora, and barbs are schooling fish and need to be in the company of their own kind. This means if you want to add one species of tetra to your tank, you should add at least 6 individuals of that same species. Having less than 6 usually results in these fish being aggressive towards any other fish in the tank.

Tetra look much better in large schools (nikkorsnapper @ flickr)

Cichlids in general are aggressive. Different species can be housed together in specific setups. As I am no cichlid expert, I will just leave you with the caution that each species has specific needs as far as number of other fish around and decorations in the tank; cichlids need to be heavily researched before you buy as the wrong setup can lead to one fish killing everything.

Why your tap water matters

This is something almost no new fishkeeper considers. Not all water is the same, and fish have requirements. Livebearers need hard water; and tetra prefer soft water. The beginner aquarist should always get fish that can easily live in her or his tap water. Stock the fish to your water not the water to your fish. It is possible to change your tank’s water parameters, but it is not easy. A new aquarist could easily kill his or her fish attempting this. There are beginner fish for all water types. Right now focus on keeping fish alive and healthy.

What parameters you need to know

Hardness (GH and KH) and pH of your water are most important parameters that determine what fish you can have. The GH and pH directly impact fish physiology through osmoregulation and ion exchange; the KH just determines how much the pH will change. Each species of fish has preferences as a result of the environment in which they evolved. Some have a wide range (like the beginner fish in my Best and Worst Beginner Fish article), and some have very narrow ranges.

How to find your local water hardness

In the US, many cities will post the GH, KH, and pH in a water quality report. You can often find these online. If you can’t find it there, you can call your local water treatment plant. They have this information on record. If you are on well water, the hardness should be in your well water report. Call the company that manages your well for it. There are home test kits for GH and KH, but these kits are expensive; they aren’t worth it for beginner fishkeepers.

Hardness matters to sensitive fish like discus (Jessa BC @ flickr)

While this is just a general map, it seems to be pretty accurate for water hardness across the US based on my conversations with other aquarists. Chart of city water hardness in the US. However, since it gives ranges, I still recommend you find the exact number for your source water (the water you will be using on your aquarium).

The “one-inch-per-gallon rule” and why it is bad

I often hear this rule quoted in stocking plans by people who don’t understand it. The rule says you can have one inch of fish per gallon of water in the tank. It was originally used as a guideline years ago to make fishkeeping easier to explain to the general public. No limitations were placed on it, so people thought it could be used across the board. It fails for the single reason that fish grow in three dimensions not just one. I’ll use a bit of basic math to explain why.

Let’s take a generic fish named Bob. Bob is only 1 inch long and has a certain width and height because he is a three dimensional object. To find the mass of Bob, multiply width x height x length. This gives us 1 for the mass. For simplicity’s sake we’ll say Bob needs 1*H of oxygen, and produces 1*W of waste with H and W as variables representing units of oxygen and waste.

Bob will grow and grow proportionately in all directions. When he is 2 inches long, he has also grown twice as wide and twice as tall. We multiply to get the mass: twice as long x twice as tall x twice as wide = 8 times the mass. Now Bob requires 8*H the oxygen and produces 8*W the waste compared to when he was 1 inch long.

Bob will continue to grow proportionately in all directions. When Bob is 3 inches long, he has also grown three times as wide and three times as tall as when he was a 1 inch fish. To find the mass, multiply: 3 x 3 x 3 = 27 times the original mass. Now Bob requires 27*H the oxygen and produces 27*W the waste. While length exhibits linear growth, mass has exponential growth. Mass will always grow faster than length. This is why a 3 inch fish cannot be compared to a 1 inch or 2 inch fish in terms of aquarium needs. Due to the larger volume (and therefore larger bioload) of the 3 inch fish, it will need a significantly larger tank.

Examples of good stocking plans

This is a lot to digest, so to help you get started, I have selected fish from my Best and Worst Beginner Fish article to give beginners examples of a stocked tank.

Hard water (GH 12, pH 7.5), 20 gal tank, 24 inches long

This is a great size for a beginner. Even weekly water changes of 50% would only be 10 gallons. In this size tank you can comfortably fit two schools of 6 to 8 fish each: one fish that swims in upper regions of the tank (column-swimming fish) like X-ray teta (Pristella maxillaris), bloodfin tetra (Aphyocharax anisitsi), or zebra danio (Danio rerio) and one fish that spends most of its time in the gravel (substrate fish) like the bandit cory catfish (Corydoras metae).

Soft water (dGH 4 pH 7.0), 40 gal tank

While on the large size for a beginner, it’s a good tank as it allows you to add a few more species of fish. In a tank this size you can have four species of fish: two schools of column swimming fish like glowlight tetra (Hemigrammus erythrozonus) and cherry barbs (Puntius titteya), one species of substrate fish like bronze cories (Corydoras aeneus), and a centerpiece fish such as the honey gourami (Trichogaster chuna).

I hope I haven’t overwhelmed you. I know this is a lot to take in. Just remember, when in doubt with stocking, less is more. If you don’t think you can fit that extra school of tetra in, don’t add them. Happy fishkeeping!

Pangio loaches (Rhizae @ flickr)

Works Referenced

Hemdal, Jay. 2009. "How much swimming space do your fish need?" practicalfishkeeping.co.uk Retrieved 12 July 2012.