So You Want a Fish Tank: Tank Size

How big a fish tank should you buy? Or, if you already have a tank, what does the size of the tank mean about what you can put in it?

Generally speaking extreme tank sizes (very large or very small) are hard to take care of. This is because there are two different things that make tanks difficult and they follow opposite trends.

Larger tanks are harder to take care of because maintenance takes longer. There’s more water to change, there’s more glass to clean, and there’s more of the tank bottom to siphon bits of food and feces off of. When I have to do a 50% water change on the 110 gallon tank at work I do it all through an automatic water change system attached to the faucet. Even given this it still takes enough time that I generally make sure no one is using that classroom for at least a whole hour.

However, smaller tanks are more difficult because when things go wrong they go wrong faster. Think about some of the things that could go wrong in a tank. Something could leach an acid or base (a new decoration, or new substrate) and change the tank pH. You’d need less acid or base to change the pH of a small tank. Rotting organic matter (extra food, a dead fish) could release ammonia. Again, you’d need less ammonia to bring the ammonia concentration into dangerous ranges in a small tank. Your heater could break (or the power could go out) and the temperature in the tank could drop. Again, a smaller tank will cool down faster (much faster, because of surface area to volume scaling). Generally speaking, more water equals more stability.

These two trends seem to cross somewhere around the 50-gallon range for freshwater. 50 gallons is enough water to be pretty stable, and so larger than 50 gallons is mostly just more work. (Of course, tank size also depends on what you want to keep in it. 50 gallons will be too small for some fish.) Note that saltwater organisms need much more stable conditions than freshwater ones and so saltwater tanks may continue to gain real stability benefits at larger sizes.

But I don’t have space, I need a small tank

This happens. I have a lot of tank space but I still want to stick an extra tank on my desk, and there’s not a lot of space there. Things to consider:

  1. You’ll need to keep a sharper eye on a small tank because things will, as I said above, go wrong much faster. This is also true because it’s pretty easy to “under load” a bigger tank, but small tanks are normally running at capacity. This means there’s no margin for error.
  2. Despite what lots and lots of pet stores will tell you five gallons is pretty much the minimum for a fish. Betta fish seem to be the most common targets for abuse in this regard. I consistently see 1 gallon or 1.5 gallon tanks sold for bettas, but bettas are really too large for these tanks, and it shows when you see the difference between a healthy and an unhealthy betta. There are some caveats in the five gallon limit, and there are some oddball fish that may be hard to find that can deal with very small pools of water in the wild. However, your standard pet store fish will do better in a five gallon and even some of the smaller schooling fish will really need a ten gallon1.
  3. Not everything you can put in a fish tank is a fish. I have a small tank that is just live plants and dwarf cherry shrimp. They are small, they produce very little waste, and they eat the dead plant material and the algae that grows in the tank which makes the tank practically maintenance-free. Ornamental invertebrates are often easier to keep in small tanks than fish.

What about fish bowls?

Fish bowls are basically just death-traps. For starters, how do you filter one? There are ways around this but fish bowls are a horrible design that are generally too small, waste a lot of their footprint (a straight-sided tank would hold more water for the same footprint), and are a bad shape for attaching the necessary equipment to.

I know what fish I want. How large a tank?

There are no hard and fast rules without knowing the exact species of fish, but there are two general rules to keep in mind.

First, for individual fish, a tank should be four times longer than the fish, twice as wide as the fish, and twice as deep. Active fish will need more room (one reason very small tanks aren’t suitable for most fish – many small fish are active enough that they need more distance) and very, very sedentary fish can make do with less. A handful of fish species (banjo catfish, for instance) spend most of their lives immobile and don’t need this much room. Certain fish may also care more or less about particular tank dimensions. Bottom dwelling fish may not really need twice their body length in tank height because they may never swim much above the bottom. Angelfish, which are “tall” fish, definitely need that height.

This shows the (rough) size of a tank for a particular fish species. The tank should be 4 body lengths long and 2 body lengths high and deep.
This work by Eric Butler is licensed under a Creative Commons Attribution 4.0 International License.

Second, there’s a general inch-per-gallon rule. A twenty gallon tank can hold about twenty inches of fish. Some fish have strange body shapes and so this doesn’t work (twenty inches of twig catfish is less total fish than twenty inches of cichlids) but it’s a general rule. The scaling ratios between linear dimensions and cubic volumes also means that this rule can’t possibly be applicable across the full size range of fish people buy for fish tanks and so the rule probably only works for “medium” fish. However, if you’re about to drop fish totaling forty inches into a twenty gallon tank you’re probably headed for disaster.

  1. This is because schooling fish do poorly by themselves, and once you have enough fish for a school you’ll need that extra room

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