A fish tank of the proper size is one of the very first requirements for setting up an aquarium correctly.  People often overlook its importance. Undersized fish tanks can cause many problems including but not limited to: Unstable water perimeters; quicker build-up of harmful chemicals; lack of enough oxygen; hard to keep clean; and not enough swimming room for the fish. Most of these problems can lead to the death of fish.

1. Unstable water parameters
Too little water can result in unstable water temperature, hardness, and PH. Fish dislike the fluctuation in water parameters because it can shock and even kill them.

2. Quick build-up of harmful substances
The aquarium nitrogen cycle is critical to keep a healthy aquarium. Fish constantly produce ammonia. Too little water can mean the ammonia concentration level will increase faster than the aquarium filter system can convert it. Many fish die in tiny fish tanks or bowls because there is just not enough room to install a proper aquarium filter system. Even when the filter system is supposed to be sufficient enough, the byproduct of aquarium nitrogen cycle – nitrate will still build up too fast for you to maintain a healthy aquarium.

3. Too little oxygen
Less water means less oxygen in the water. Limited water surface in a small tank also can prevent enough gas exchange from taking place between the water and the atmosphere. Fish will die from not enough oxygen.

4. Hard to keep it clean
A significant amount of fish poop and uneaten food can result in a dirtier tank if the fish tank is small. You may find yourself in a situation where the fish tank is always dirty despite your best effort to clean it frequently.

5. Not enough swimming room
All fish need a certain amount of swimming room. If they can’t move around freely without bumping into each other or onto the side of the tank, they will become stressed. Stressed fish will have a weaker immune system which might result in catching diseases or even death.

Now we know the terrible consequences of having a small fish tank.

The question now is – How big of a fish tank is big enough? What size of a fish tank do you need?
You may have two different approaches. One, you may get a fish tank first before you decide on what fish species and how many fish are suitable for it. Two, you may research the fish species first and make up your mind on what fish you will get, and then get a fish tank with the size suitable for your plan.

How big of a fish tank you should get is not only determined by your available budget, space, but it should also be decided by what species and the quantity of fish you plan to have. Each fish species has its minimum requirement for the tank size. There are hundreds if not thousands different pet fish species raised by people as a hobby. There is no need to get the exact number for required tank size on each of them. While it is true the tank size should be determined by the fish species; there are always some general rules which can be applied in most cases.

Rule #1 – Stay away from fish bowls and any fish tank under 5 US gallons in size!
It is highly recommended to get at least a 5-gallon fish tank for even just a single Betta fish (also known as Siamese Fighter Fish). In fact, we recommend a 5-gallon as the bare minimal tank size for even the smallest fish species. Anything smaller is not ideal to keep fish healthy due to the fact it will be harder to keep the water parameters stable.

Now you may wonder why there are fishbowls of one, two, and three gallons. There are smaller than 5-gallon fish tanks and fish bowls in the stores; they are in fact unsuitable for fish at all. It is all about marketing and profit without caring about the well-being or the basic living condition of the pet fish in mind. It is hard to keep fish alive in those tiny fish bowls. Therefore, we recommend staying away from them. The shape of the fishbowl is also awkward. It has a smaller water surface area for gas exchange. It also means less oxygen for the fish. Moreover, the small size of fishbowl does not have enough room for proper installation of equipment necessary for the survival of the fish.

Rule #2 – For the fish tank size, the bigger the tank, the better!
There is no such thing as too big when it comes to the size of a fish tank. You should get the biggest fish tank you are willing to pay for if you have space at home. From the economical point of view, a basic 5-gallon glass fish tank goes for only $11 in most local stores, and a 10-gallon glass tank is just $14. The prices might be different depending on where you live, but it can’t be far off. Obviously, a 10-gallon is a better choice as the lowest budget aquarium if you have space at home for it. With adequate filtration, a 10-gallon fish tank should be fine to house up to 5~6 tropical fish species of less than 3 inches in length.

If you have more space available at home as well as the budget, go for a 20- gallon fish tank or even a 29-gallon fish tank as your first fish tank. These tanks are still considered small-sized tank by aquarium standards, and they are still cheap to get. With a bigger fish tank, you will also have more options when it comes to stock it with fish. A 29-gallon tank should be enough for most small and medium-sized fish up to 6” long. Please note that these numbers are not the absolute rule.

Some of the starter kits with most things included are fairly cheap and easy to set up.
10-gallon aquarium kit
20-gallon aquarium kit
The article of How to set up an aquarium will tell more details on what equipment you need to set everything up.

(False) Rules of Thumb on Fish Tank Size

The inch per gallon (false) rule
There are (wrong) rules regarding the fish tank size and fish stocking. One of the most common ones is the “inch per gallon rule.” It is (wrongfully) said that you should get no more than a fish of one-inch-long per gallon of water. This is wrong because a lot of fish species have different body mass and appetite even when they have the same length. One extreme example is the goldfish. They can weight 5~10 times as much as a tropical fish of the same length. With more body mass, they produce more waste and thus requiring a much bigger fish tank. While you should not follow this false rule, it has to be noted that it might not be too far off for most tropical fish species of less than 3 inches long.

The case of goldfish is extreme. They are incredibly messy and will grow to be huge quickly. A minimal 20-gallon fish tank for just one Fancy Goldfish is highly recommended and no less than 30 gallons for two of them. A Common Goldfish requires a minimum 40-gallon fish tank for the first one, and you need a 55-gallon for a pair. It is essential that you get the tank size above the bare minimum requirement, or you will have a problem to keep the ammonia level at bay.

The fish will not outgrow the tank (false) assumption
There is another wrong assumption that fish will only grow to be as big as the tank allows. Some people say the fish will stay small if the fish tank is small. This assumption is wrong! There is entirely no reason for fish to stop growing just because the fish tank is not big enough. When the tank is too small for a particular species, the fish will outgrow the tank and make the tank maintenance hard to keep up. It might cause ammonia spike which can kill the fish. You must use the full adult size instead of the current size of the fish to measure the fish tank size requirement. Do not get a small tank just because the fish is still young. Unless you are certain that you will get a bigger fish tank very soon.

Fish tank length requirement
Another general unwritten rule is the tank length should be at least seven times longer than the fish. So if you have a Betta fish of 2.5” long, you need a tank with at least 17.5” in length. This rule is false. Some fish such as slow-moving Betta do not move around very fast. Thus they will do fine in a smaller tank than some fast moving fish such as Zebra Danios. Under normal circumstances, more active and fast swimming species require more swimming room.  Therefore, they need a bigger fish tank than the timid fish species.

Some important points to pay attention to when it comes to the fish tank size

School Fish
Some fish such as Tetra, Rasboras, Corydoras are school fish. Most school fish species need a minimum of 5~6 in each group. Not having enough of them in the same aquarium can result in their being shy and stressed, thus not displaying their natural behaviors. When you plan to get a certain species of school fish, you should keep the minimum recommended number in mind. More fish will obviously require the more swimming room as well as overall water volume to sustain them.

Fish Tank dimension
Certain fish species have a “weird” shape. Angelfish, for example, has a very tall body shape comparing to most other fish of the same length. They require those taller dimension tanks. (Not all tanks of the same size have the same dimensions). Another example is that many species of catfish need to go to the surface for air.  As a result, they like shallow tanks. 

As long as the bare minimum is met, there is no absolute rule or any magic number for how to decide on the size of a fish tank. You should get the biggest fish tank you are willing to pay for if you have space at home. Just use your common sense when it comes to stocking your aquarium with fish. Some research on the requirement of individual fish species might be helpful if you are already sure on what fish you want to get.