Koi (Cyprinus carpio carpio) Care

Basics

Koi (Cyprinus carpio carpio) are a common sight in ornamental ponds around the world. They represent good fortune and luck in many cultures and are sometimes referred to as living jewels. Koi are the domesticated form of wild Asian carps. The processed was begun in the 1800’s when fish that showed more gold coloring than others were pulled from fisherman’s net and put into ornamental ponds. By the 20^th century many of the recognizable color patterns were firmly established.

Koi come in combinations of yellow, orange, red, black and white. One of the most common color patterns is a simple white and red koi. This is called kohaku. A kohaku koi with a single red patch on its head is called a tancho and is highly prized because they resemble the Japanese flag as well as the sacred tancho crane. Some tancho koi sell for over a thousand dollars.

Tancho koi (KoiQuestion @ Flickr)

Minimum tank size

As beautiful as they are, koi are not for the everyday hobbyist. They are large fish that require large tanks or ponds. The average size of a properly cared for adult koi is over a foot (30 cm) long. When given enough room and good food, champion koi will reach almost three feet (91 cm) long. Like goldfish, they are also copious producers of ammonia. A fish of this size and dirtiness cannot be housed in a normal sized tank. I don’t recommend anything smaller than 500 gallons (1,892 L) for a single koi. I know other people would choose to keep them in smaller tanks, but I feel it would not give the fish enough room to reach its full potential.

There are a few different ways to stock a pond. Some koi hobbyists have heavily stocked ponds with professional-grade filtration systems and do large weekly water changes. Some, like myself, chose a more relaxed approach. I only have one fish per 550 gal (2,082 L) of water. This lower stocking level means I don’t have to spend as much on equipment and maintenance. As with a lot of fish, there are a few right ways to stock a pond and most of it depends on how much money and time you want to put into it. But there is one certainty with koi: they need large ponds.

Temperature range

Koi are a hardy pond fish with a wide temperature range. They can survive in water that is almost below freezing or up to tropical temperatures. The important thing is that they experience these transitions gradually. In a large enough pond there is little worry of temperatures changing drastically overnight; this is another reason to keep koi in large volumes of water. The generally accepted range of temperature tolerance for koi is 35F (2C) to 85F (30C). I highly recommend putting a thermometer into the pond because air temperature and water temperatures can differ greatly.

Keeping the water moving helps prevent freezing over (Aquascape Inc)

The depth of a pond plays a large role in temperature control. The deeper the pond is, the cooler it will be in the summer and the warmer it will be in the winter. This may seem counter-intuitive, but the ground acts as insulator against both the cold and the warm. It is this same principle of insulation that keeps caverns the same temperature year-round. The farther north you live, the deeper you will need to dig your pond in order to prevent it from freezing solid in the winter. In the southern US you may only need to go down 2.5 feet (76 cm), but in Ontario, Canada you may have to dig down 5 feet (1.5 m) deep.

Water quality

Just because koi are hardy doesn’t mean that water quality isn’t important. If you want to grow large and beautiful koi, clean water is a must. Just like with tanks, koi need filters and water changes. The filters on ponds are large and scaled to the pond, but just like tanks they will need some kind of regular maintenance, whether it is professionally done or your own labor. I clean the debris out of my filter about three times a year: once when I take the leaf net off in the spring; once in the summer; and once in the fall before I put the leaf net on the pond for the winter.

Because ponds are located outside we get assistance from Mother Nature with water changes. Unless you live in a desert climate, water changes are taken care of for you. When it rains, the fresh water enters the pond and causes it to overflow (most ponds have directed overflow drainage that also protects the fish from getting swept out). When the pond overflows, the older dirty water flows out and is replaced with the incoming fresh water. These pond overflows can also provide a nice area to plant a bog garden.

Bog garden with pitcher plants adjascent to koi pond (mmwm @ Flickr)

Food

Koi, like goldfish, are omnivorous fish. Because they are subject to large temperature changes throughout the year, their diet has to change with the seasons. Fish are cold-blooded which means their ability to digest food is dependent on the outside temperature. In cool weather they cannot handle protein-rich foods; if fed too much it can rot in their digestive tract and kill them. For this reason there are two general types of koi food sold: warm water food and cool water food.

At temperatures above 65F (18C) koi should be fed a staple food, sometimes called summer feed. This is food with a higher protein content, usually above 35%. Koi metabolisms work fast enough at these temperatures to digest proteins. This is when most growth occurs. As the temperature increase, you increase both the volume of food and the frequencies of feeding.

Koi eating floating pellets (Kristen Shoemaker @ Flickr)

Below 65F (18C) koi should be fed a wheat germ-based food. Below 65F (18C) the koi metabolism slows and cannot handle high proteins, but until the water reaches 50F (10C) koi still need to be fed. This high-carb food is easy to digest and helps them gather enough energy for winter hibernation. As the temperature decreases you lower the amount and frequency of feedings until you are only feeding once every couple of days. Below 50F (10C) your koi should not be fed. They are in hibernation. If the temperature warms slightly in the winter, they will graze on algae and detritus in the pond. 

I have included below the temperature feeding schedule I use for my koi. There is some debate about the exact temperatures among koi hobbyists but this is what has worked for me.

TEMP (°F)	FEEDING REGIMINE
Above 86	Feed cautiously as adult koi metabolism can shut down in temperatures approaching 90F
70 – 86	Every day. As much as they can eat in about 10 minutes. Feed 3 – 4 times a day.
69 – 65	Every day, twice a day with a staple food.
64 – 61	Every day, once a day (mix of wheat germ and staple). Midday feedings are best.
60 – 55	Every 3 to 4 days (Wheat germ base)
54 – 50	Feed once a week with presoaked if koi are moving around (Wheat germ base)
Below 50	Do not feed!

 

Substrate

This is more of personal preference than koi preference. Koi thrive in bare-bottomed ponds, gravel-bottomed ponds, and mud-bottomed ponds. Bare-bottom ponds are easier to keep clean. Most serious koi hobbyists use these types of ponds because it enables them to keep the water crystal clear to grow massive koi. Some Japanese water gardens use mud bottom ponds because that is traditionally how koi were kept. Koi breeders who breed the traditional way use mud bottom ponds as spawning sites. The downside to mud-bottom ponds is that koi like to root around in the substrate, and the stirred-up mud obscures your view of these gorgeous fish. I prefer a light layer of large gravel over the pond liner as it produces a natural look without the cloudiness of mud.

Tank mates

Koi, like most carps and minnows, are social fish. They prefer the company of their own kind but also do well with goldfish (Carassius auratus auratus), golden tench (Tinca tinca), channel catfish (Ictalurus punctatus), and golden orfe (Leuciscus idus). Sturgeon (Acipenseridae) do not make good pond-mates for koi because they are shy and require a carnivore-based diet. Koi should not be housed with fish that will fit in their mouth because they can and will eat the smaller fish. While guppies (Poecilia reticulata) would look pretty in with your koi, they will likely become snacks.

Juvenile blue and golden orfe (Pete Turnbull @ Flickr)

Plants

Koi, like goldfish, are notorious for eating plants in ponds. Most people don’t keep floating or submerged plants in koi ponds for this reason. However, marginal plants, bog filters, and waterfall filters are very popular. Not only do they add an aesthetic beauty but they also help keep the water clean.

Marginal plants are placed on the edge of the pond so that just the roots are in the water. There are two types of marginal plants: hardy and tropical. Hardy marginals can survive through the winter in most places but tropical marginals cannot (unless you live in a semi-tropical climate). Most of these plants need a decent amount of sun, so in my mostly shaded pond I can only grow a few marginals. I maintain a number of yellow flag iris (Iris pseudacorus). Tall marginals like cattails (Typha spp) and arrowheads (Sagittaria spp) should be placed on the far side of the pond so as not to obstruct your view. Low-growing marginals like marsh marigolds (Caltha palustris) should be planted on the side of the pond where you will be viewing the pond most times.

Marginal plants can also be put into a bog filter. This is a separate area of the pond that the koi cannot reach. It will only have about 6 inches of water which enables the roots to stay wet. This water comes from the koi pond filled with nutrients for the plants. The plants remove the nutrients generated by the fish and the cleaned water is returned to the pond. Some people who have koi that even eat the roots find this a wonderful way to introduce plants into the system without having to worry about them becoming a koi snack.

Water hyacinth in waterfall return (source unknown)

Another way koi-keepers can sneak plants into a koi pond is with a waterfall filter. The water return at the top of the waterfall of a koi pond is often an open-top barrel. Some people (including myself) use this space to grow plants. You can use floating plants like water hyacinth or marginal plants, and I’ve even seen people grow veggies hydroponically there.

Interesting facts

Koi are renowned for their longevity. The average lifespan of a properly cared for koi is between 20 and 35 years, but there have been some special cases that long surpassed that average. In a small, creek-fed pond in Gifu, Japan there were six fish that far surpassed that average. Hanako, a scarlet koi, was the oldest of the lot. She was hatched in 1751. To put that in historical perspective she hatched 25 years before the Declaration of Independence was signed. This was verified by Prof. Masayoshi Hiro, D.Sc., Laboratory of Domestic Science, Nagoya Women's College. She removed a scale from Hanako’s body and counted the growth rings under a microscope, much as you would count the growth rings of a tree. In 1966 when the interview with Hanako’s owner was aired on radio, the other koi in the pond were 170, 155, 151, and 141. Unfortunately Hanako is no longer alive today. She died July 17, 1977 and was 226 when she died.

Hanako and her keeper (Northernkoi.org)

All over the world, koi symbolize luck and fortune, but most people don’t know the reason why. An old Chinese legends hold that long ago the supreme god of the heavens called the Yellow Emperor became displease with the humans for their evil ways. He ordered the rain god to flood the earth to destroy the humans. Kun, the Yellow Emperor’s grandson was distraught by the destruction and pleaded with his grandfather to spare the humans. The Yellow Emperor did not relent, so Kun stole some “magic mud” from his grandfather’s treasure. Kun sprinkled this mud and as it hit the water turned into land where the humans could take refuge. Furious at his grandson, the Yellow Emperor sent the fire god to kill Kun who tried to disguise himself as a white horse, but the fire god saw through it and struck Kun down.

After many years of continued flooding a golden dragon named Yu emerged from the white horse that was once Kun. He flew to the heavens to plead with his great-grandfather to stop the flood. This time his perseverance paid off, and the Yellow Emperor told Yu to take the magic mud and make land again. Yu easily carved a river through the soft mud, but when he came to rocky cliffs he had to greatly increase his effort to dig out a gorge. In doing so he created huge waterfalls hundreds of feet high. He declared the waterfalls sacred to dragons. The river eventually became known as the Yellow River and those 12-mile long gorges located in China’s Shanxi province became known as Dragon’s Gate. Every spring thousands of koi swim up the Yellow River and if they have the skill and perseverance of a dragon to climb the falls they become dragons themselves. Because of this legend, koi are symbols of hard work, patience, and skill which are all things needed to have good fortune in life.

Taking care of koi isn’t always an easy task, but just like Yu and the Dragon’s Gate waterfalls persevering through the hardships can make the final outcome that much more beautiful.

Works referenced

"Frequently asked water gardening questions." Crystal Creek Pond Supply LLC. www.pondkoi.com. Updated: 8 May 2013. Retrieved: 8 June 2013. 

"Koi Encyclopedia." Pan Intercorp. www.koi.com. Updated: 8 June 2013. Retrieved: 8 June 2013.

"Legend of Dragon's Gate." www.foolonthenet.blogspot.com. Updated: 8 November 2010. Retrieved: 7 June 2013.

"The Story of Hanako." Koi.net. www.playkoi.com. Updated: 29 January 2013. Retrieved: 8 June 2013. 

"Temperature." Koi Club of San Diego. www.koiclubsandiego.org. Updated: 7 June 2013. Retrieved: 8 June 2013.

**Disclaimer** Since there is little to no scientific research about fishkeeping published, much of the hobby today is opinion, but they are opinions given by keepers and breeders who have been doing this longer than some of us have been alive. Temperature ranges and adult sizes are not opinion. They have been scientifically documented.

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.