How many fish can I keep in my aquarium?

We admit that we are more conservative in our guidelines for stocking aquariums than other information sources. There is a good reason for this. In the thousands of aquariums we've seen over the years, we have consistently seen the highest rate of failure and problems in those aquariums that were over-stocked. Some "aquarium experts" (we use that term with tongue-in-cheek, as we believe there really is no such thing), may not agree with these rather conservative guidelines, but they have worked best in our experience.
Freshwater: A good rule of thumb is one inch of fish per gallon of water, but there are exceptions. The guideline of an inch of fish per gallon of water is based on a standard rectangular aquarium. If you have an unusually shaped fish tank, such as a hexagonal or extra-high aquarium, try keeping one inch of fish for every 12 square inches (length times width) of aquarium surface area. Remember to allow for growth by stocking your aquarium only to 75% of it's total capacity.
Saltwater: Saltwater has far less dissolved oxygen than freshwater due to higher density. The rule that we at The Aquarium Professionals Group use for saltwater tanks is three inches of fish for every square foot of surface area. For example, a 55-Gallon aquarium (48" Long by 13" Wide) has approximately four square feet of surface area, so approximately twelve inches of fish can be kept in a 55-gallon aquarium. In saltwater, look for quality, not quantity! There are exceptions. Most eels are long but skinny, so using 1/4 to 1/3 of most eel’s body length as a measure is safe. A large angelfish is almost as tall as it is long, and puffers are wide in girth, so using 1-1/4 of these fish’s body lengths as a measure is probably more accurate. Stock your aquarium at 75% of it’s total capacity to allow for growth. Increasing or improving the aquarium filtration used can also increase the number of fish that can be kept, but use caution here. It is better to add more filtration to improve the environment so your fish live longer, rather than adding it to enable you to keep more fish.

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What other factors affect the number of fish kept in an aquarium?

The maximum number of living animals that may be safely kept in a closed environment such as a fish tank is usually referred to as the "carrying capacity" of that system. In science, the carrying capacity of a closed environment is usually expressed in units of body weight (mass). The total weight of all of the animal, plant and microbial life in a given environment is called the "biomass."
Biologists can calculate a close approximation of the carrying capacity of any closed aquatic system by measuring the levels of dissolved gases and metabolic organics in the water and calculating the oxygen consumption and waste production of the animals to be kept. They then use these values in mathematical formulae to determine the maximum mass of animal life the closed environment can support. The behavioral and physiological needs, as well as the waste production of the desired animals to be kept, are also factored into other calculations to arrive at an even more precise value.
We aquarium hobbyists don't usually get quite that involved, and we certainly don't weigh our fish. We rely on water test results, common sense, books, and practical experience to help us determine how many fish we can keep in our aquariums. The carrying capacity of an aquarium is influenced by several factors, some of which are dependant on others. The most important of these is the amount of dissolved oxygen available to the fish and nitrifying bacteria form the water they live in. Oxygen levels in aquarium water are dependant on the surface area of the tank, the temperature and density of the water, the amount of solid waste and detritus that is allowed to accumulate and decay in the environment, how much and how often the water is changed, and the amount and types of filtration being used.
The size of the beneficial bacteria population in the aquarium is another factor that influences how many fish may be kept. These bacteria consume harmful nitrogenous fish waste and convert it into less harmful by-products that may be removed by water changes or de-nitrifying bacteria. Without a healthy population of these micro-organisms in the aquarium system, the fish would be poisoned by their own waste. The bacteria are usually cultivated in some type of biological filter attached to or in the tank, such as an under gravel or wet/dry filter. In a Berlin or Jaubert system reef aquarium, or a so-called "balanced" live-plant freshwater tank, the beneficial bacteria inhabit the rock and substrate in the aquarium. Nitrifying bacteria require oxygen to metabolize fish waste, and their population is influenced by the levels of dissolved oxygen in the aquarium water. This can present a catch-22 situation in some heavily-populated aquariums, as the bacteria and fish are in competition with one another for oxygen, but the fish cannot live without the bacteria.
There are other issues that need to be considered when trying to determine how many fish may be kept in an aquarium. At The Aquarium Professionals Group, we call these the "stress-factors." These are aspects of an aquarium or it's inhabitants that cause stress to one, several, or all of the fish in the aquatic environment. They are:
1) Adverse water condition problems beyond the control of the aquarium owner. An example of this might be high water temperature in an aquarium that cannot be equipped with a chiller due to space or budget limitations.
2) Compatibility, territorial, and other behavioral issues. Many fish may be natural enemies of other fish in their natural environment. Some fish have large territories in the wild which they will defend against other fish who stray into their "space." In some cases this territory might be the entire tank! Still other fish shoal (or school) with others of their own kind in the wild and keeping single specimens of these may result in uncharacteristic behavior or stress in that particular species.
3) Nutritional and other environmental dependency issues. Some fish may require a particular food or habitat in order to survive which may be lacking in the aquarium or is unavailable to the aquarist. In some cases, changing the habitat or supplying a particular food to accommodate these species will solve these problems. In other cases, that species of fish should not be kept in captivity!

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If I use an air pump to aerate my aquarium, can I keep more fish?

It is a misconception that adding air bubbles through an air pump to an aquarium directly adds oxygen to the water. The air we breathe is mostly nitrogen, not oxygen. Air bubbles do not dissolve in the tank, but rise to the surface and break, causing the water surface to ripple. This increases the surface area of the aquarium through agitation, and thus aids in the exchange of gasses at the water's surface, but not enough to add a lot more fish. We highly recommend the use of aeration to aid gas exchange in marine aquaria, especially reef aquariums, but not for the purpose of adding more fish.

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How does the shape of my aquarium affect the number of fish I can keep?

While there are other factors that determine how many fish can be kept in a given aquarium, the most important is the level of dissolved oxygen in the aquarium water. All aquatic animals in an aquarium require oxygen to survive. The higher the level of dissolved oxygen, the more fish the aquarium will hold. Dissolved oxygen levels in an aquarium are directly related to the surface area of the tank (the amount of water that is exposed to air). It is at the aquarium's surface that carbon dioxide escapes and oxygen enters the water. Aquariums that are wider and/or longer will have more surface area and will therefore hold more fish. As a general rule, standard rectangular aquariums will usually hold more fish than an unusual-shaped aquarium of the same volume. For example, a 30 gallon rectangular tank measuring 36" long x 13" wide has approximately 3 square feet of surface area. A 30 gallon hexagon aquarium measuring 22" long x 16" wide has approximately 1¾ square feet of surface area. In this example, the rectangular aquarium will hold nearly two times more fish than the hexagon aquarium.

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I have a Betta in a 20 gallon tank. Can I put Oscars with him?

No. Oscars are Cichlids that grow quite large and they need much more room. They have large mouths and will gladly eat your Betta, even when they are smaller. A 55 gallon tank would be recommended for Oscars.

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I have a 20 Gallon Saltwater Hexagon Tank, and therefore have a reduced suface. What kind of fish would you recommend to have? I want 2 Big Lionfish or 2 Porcupine Puffers. What do I need just to get the tank balanced?

A 20 gallon hexagon tank has the same surface area as a ten gallon aquarium. It has roughly one square foot of surface area. That means you can keep a TOTAL of three inches of fish.

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Could you give me some information about how to raise baby Convict Cichlids? We just had a hatch and nmeed to know how to rear the fry.

Keep the tank clean because over-feeding will occur when trying to provide enough food for the babies. You should be careful when cleaning the tank to avoid getting too close to the baby fish, or the parents may get frantic and start to eat the babies.

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I have had my eye on a porcupine puffer. I have heard that they are venomous and agressive. Will it harm my other fish? What should I feed it to keep it happy?

Porcupine puffers are only venemous if you EAT them. They are not aggressive at all, but they can become fin nippers if kept with peaceful fish with long attractive fins. Typically these puffers make good community fish, but only for large tanks. They grow quite large and do so very rapidly. They enjoy prepared whole frozen fish foods like krill, clam, mussel, squid and silversides.

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had a 55 gallon saltwater tank a few years ago which I am just starting to set up again. Originally I had 2 eels (1 snowflake and I'm not sure what the other one was) and 2 groupers (1 panther and 1 red one). They all lasted for about 7 years. Is that the normal lifespan for most saltwater fish? I want to set up a new 55 gallon tank and am planning on getting a few groupers and at least four moray eels. Can I put some crabs and shrimp in with those fish?

If you keep slow-moving carnivores like eels and groupers, I would recommend stopping at four for a 55 gallon tank. Those fish grow rather large, and both groupers and eels live many years. Eventually they get too large for the tank and slowly wither away due to lack of oxygen. For some species, keeping more than one or two in a 55 gallon tank would not be a good idea. No, it would not be a good idea to keep crustaceans in with eels or groupers. They'll just be expensive snacks.

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Recently I bought a 29 gallon tank for my three goldfish. I had read that it is 10 gallons per goldfish as a general rule. I figured what was one gallon. But then I came across a few other places saying you need 30 gallons per goldfish. Will the 29 be sufficient for three?

We think you'll be fine. The rule we use is one goldfish per 5 gallons MINIMUM. Ten gallons is even better for these great fish that create a big mess and get very big very quickly.

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Frequent Aquarium Questions