How big do aquarium fish get? Fish size and tank size explained

Most fish in a shop are babies. A 5 cm (2 inch) pleco or oscar in a store tank can grow into a fish the length of your forearm. The single most useful thing you can do before buying any fish is look up its adult size, and make sure your tank can hold that adult, not the baby in the bag.

Every species card on this site shows the adult size in centimetres and inches. Use that number, not the size in the shop, to choose a tank and decide how many fish fit. And treat the old "one inch of fish per gallon" rule as a rough idea for small fish only: long, bulky and territorial fish need a tank sized for their length and footprint.

How to check it

Look up the adult size for every fish you keep or want, on its species card. Write it down in cm.
Measure your tank: length, front-to-back width and height, inside the glass. Volume alone doesn't tell you enough.
Compare the biggest fish with the tank. A simple test, used by one long-running cichlid site: the tank should be at least about 5 times the fish's adult length from end to end (about 4 times for calm community fish, and more for fast swimmers and aggressive fish), and at least about 1.2 times its adult length from front to back, so it can turn round easily. Thick-bodied fish need a little more width.
Add up the whole group. Ten small fish are very different from one big one, but they all add waste.
Watch how your fish behave. Fish that bump the glass when they turn, pace up and down, or fight more as they grow are telling you the tank is getting small.

When it's a problem

Normal: every fish has room to turn and swim, the water tests clean between water changes, and fights are rare.
Plan ahead: you have young fish whose adult size will be too big for the tank. You have time, but start planning a bigger tank or a new home now.
Serious: fish that can barely turn around, damaged fins or snouts from hitting the glass, constant fighting, or nitrate that climbs fast after every water change. These are signs of a tank that is too small or too full.
Urgent: fish dying, gasping or being killed by tank mates. Test the water and see fish dying, gasping or aggression.

What to do right now

Look up every fish's adult size and compare the biggest with your tank's length and width.
Test the water for ammonia, nitrite and nitrate. A tank that is too full shows it in the water first.
Stop adding fish until you know the tank can hold the adults you already have.
Do a water change if nitrate is high, and keep up with regular ones while you plan.
Don't buy a fish "to keep it small". A small tank doesn't make a healthy small fish (see stunting below).

What to do next

Why "1 inch per gallon" is only a rough guide

The rule says you can keep one inch of fish for every US gallon of water (about 1 cm for every 1.5 litres). Nobody seems to know where it came from: the author of Aquarium Science says he has no idea. It can work as a rough check for small, slim community fish, but it breaks down quickly:

Big fish are much heavier than small ones. A 25 cm (10 inch) fish weighs far more, eats far more and makes far more waste than ten 2.5 cm (1 inch) fish. The rule counts them the same.
Length matters, not just litres. A tall, narrow tank can have plenty of litres but not enough room for a long fish to turn.
Behaviour matters. Territorial fish need space to claim. Fast swimmers need length. Schooling fish need room for the whole group.
Filtration matters. A well-filtered tank with regular water changes can hold more small fish than a tank with a tiny filter.

The cichlid site linked below calls the rule useless for most fish. A better approach is to size the tank for the biggest fish's length and width first, then add smaller fish slowly while testing the water.

Fish that outgrow most tanks

These are sold cheaply and small, but grow very large. The figures below are the maximum lengths recorded on FishBase; many aquarium fish stay smaller than the wild record, but not by enough to fit a small tank.

Fish	Maximum recorded size	Why it's a problem
Common or sailfin pleco (Pterygoplichthys)	About 58 cm (23 in)	Sold as an "algae eater". Makes a lot of waste and needs a very large tank. Released fish now live wild in countries far from their home range.
Oscar	About 46 cm (18 in); a more typical adult is about 24 cm (9.5 in)	Grows fast, is thick-bodied and messy. Even a typical adult needs a tank of roughly 120 cm (4 ft) or more by the 5 times rule.
Clown loach	About 30 cm (12 in)	Lives in groups, so in time you need room for several big fish.
Iridescent shark (a catfish, not a shark)	About 130 cm (51 in)	A schooling fish that no home aquarium can hold as an adult.

For plecos that do stay small, such as bristlenose or zebra plecos, see what do plecos eat and the species cards.

What about stunting?

A fish kept in a small tank often stops growing at a smaller size. This is called stunting, and it isn't a sign the fish has "grown to fit the tank". A vet writing about goldfish explains that it mostly comes from poor conditions: a small tank is hard to keep clean, nitrate builds up, the fish is stressed, and growth slows. He suggests nitrate above about 20 ppm harms long-term growth and health. Scientists don't yet agree on everything stunting does to a fish, but there's no reason to aim for it. Give the fish room and clean water instead.

How to stock by adult size

Pick your biggest fish first and check the tank is long and wide enough for it as an adult.
Add up the adult sizes of everything else you want, and be honest about how big they will get.
Leave space for groups to grow, for fish to breed, and for the odd fish that grows larger than average.
Spread fish through the tank: some at the top, some in the middle, some on the bottom. The species cards show where each one swims.
Add fish a few at a time, in a cycled tank, and test the water before each new batch. Aquarium Science suggests watching for cloudy water and waiting until it's clear before adding more.
Check predators. A big fish will eat any tank mate that fits in its mouth, so adult size matters for tank mates too. See aggression.

If a fish has outgrown its tank

Upgrade the tank if you can, before the fish is fully grown.
Rehome it. Ask local fish stores, aquarium clubs and public aquariums. Large fish are hard to rehome, which is why it's best to check adult size before buying.
Never release a fish into a river, lake or pond. It's cruel to the fish, often illegal, and it harms wild fish. Released sailfin plecos now live wild in many countries far from South America.

Products that help

A bigger tank than you think you need: larger tanks are more stable and give you room for mistakes. Long, wide tanks suit most fish better than tall ones.
A filter rated for more than your tank's volume: more filtration copes better with growing fish.
Liquid test kit for ammonia, nitrite and nitrate: tells you when a tank is getting too full.
Tape measure: measure the inside of the tank, and measure fish in a clear container if you're unsure how big they are.
Water-change siphon and buckets: regular water changes matter even more as fish grow.
Caves, rocks, wood and plants: break up sight lines so territorial fish can each claim a space.

How to prevent problems

Research the adult size before you buy, every time. Use the species cards.
Buy the tank for the adult, not the baby in the shop.
Be wary of fish sold as "tank busters", "monster fish" or very cheap algae eaters unless you have a very large tank.
Don't trust "it'll only grow to the size of the tank". It won't stay healthy that way.
Keep water changes regular as fish grow, and test nitrate.
Plan for long lives. Big fish often live for many years; see how long fish live.

Why it matters

A fish's tank is its whole world. Too little room means more waste in less water, more fighting, more stress and more disease. A marine aquarist writing for Reef Builders notes that fish in tanks that are too small can end up with bent fins, bruised snouts and damaged eyes. And a fish that outgrows its home often ends up rehomed in a hurry, or released, which is how sailfin plecos became pests in rivers far from South America. Checking adult size takes a minute and prevents all of this.

When to get help

Ask for help if you aren't sure how big a fish will get (some are sold under vague names), if a fish has outgrown its tank and you need a new home for it, or if your tank's water keeps going bad even with regular water changes. Local fish stores, aquarium clubs and breeders can often tell you how big a fish really grows in home tanks.

In the game

Fish size is one of the main rules of Multiple Tank Syndrome, and the game deliberately uses the simple "inch per gallon" idea because it is easy to count at the table. Each tank card has a size in gallons, each fish card shows its adult size in inches, and a tank holds as many inches of fish as it has gallons. Size matters in other ways too: a predator eats any tank mate a third of its size or smaller (Java moss hides fish of 2 inches or less), and when a tank gets too full, aggressive and territorial fish start killing smaller tank mates. Velvet kills fish 2 inches or smaller, hole-in-the-head kills fish 6 inches and up, and the Monster tank goal needs 40 inches of fish in one tank. In a real tank, think about length, width and waste, not just inches per gallon.

Sources

MalawiCichlids.com. Tank size and fish length (5 times length, 1.2 times width, "inch per gallon" rule).
Aquarium Science. Stocking (adult size, adding fish slowly, filtration).
RSPCA Australia. How should I care for my tropical fish? (plan for adult size and numbers).
Javed L, DVM. Stunted goldfish growth: how it happens. Hepper.
FishBase. Pterygoplichthys pardalis, Astronotus ocellatus, Chromobotia macracanthus and Pangasianodon hypophthalmus (maximum lengths, introduced range).
Kurtz J (2014). How to estimate the right aquarium size for any given fish species. Reef Builders (room to turn, injuries in small tanks).