How many fish you can put into a tank relies on more than the old “inch per gallon” rule. A 10 gallon tank can house 8-10 small adult fish; 20 gallon, 16-20 small fish; and 55 gallon, 50-60 small fish. The reason this works for small fish is because small and slim adult fish (1-1.5”) have a relatively low bioload. But this rule does not work for larger, fully bodied fish.
How Many Fish? Quick Stocking Guide by Tank Size
Why the “Inch Per Gallon” Rule Doesn’t Work
The “inch per gallon” rule doesn’t work because it doesn’t take into account adult size, waste production, or swimming space & tank dimensions. You couldn’t put a 20” fish in a 20 gallon tall tank. It’s more complex than that.
What Actually Matters for Stocking
Adult Size (Not Current Size)
When you buy fish at a store or online, you are typically purchasing young fish. They have a lot of growing to do. That cute little 3” oscar will grow into a 10-14” heavy bodied fish with a substantially higher bioload.
Waste Production (Bioload)
A slim danio or rainbowfish is going to produce less waste than a chunky fancy goldfish. 1 inch of a tetra is not the same as 1 inch of goldfish. A good stocking estimator needs to take bioload into account.
Swimming Space & Tank Dimensions
Imagine two fully grown angelfish at 6” long (and 6” tall!) swimming around a 20 gallon long tank. Yes, they can swim left and right, but have very little space vertically. Also consider the surface area difference between a 20 long and a 20 tall. The 20 long has way more surface area, which is very important for oxygen exchange.
Stocking Examples by Popular Tank Sizes
10 Gallon Tank Options
- Option 1: 8-10 neon tetras + 3-4 pygmy corydoras
- Option 2: 6 guppies + 4 cherry shrimp
- Option 3: 1 betta + 3 pygmy corydoras
- Option 4: 6 ember tetras + 4 chili rasboras
20 Gallon Tank Options
- Option 1: 10 neon tetras + 6 bronze corydoras + 6 shrimp
- Option 2: 8 guppies + 8 platies + 4 bronze corydoras
- Option 3: 2 angelfish + 8 red eye tetras (20G tall)
- Option 4: 1 betta + 10 neon tetras + 6 bronze corydoras
55 Gallon Tank Options
- Option 1: 20 neon tetras + 12 corydoras + 6 platies
- Option 2: 4 angelfish + 15 red eye tetras + 8 bronze corydoras
- Option 3: 12 rainbow fish + 10 cherry barbs + 6 yoyo loaches
Calculate Your Exact Tank Capacity
There are generally 3 methods to calculate tank capacity. The first is the outdate “inch per gallon” rule. The second is a similar calculation that uses the tanks surface area. The most accurate and safest method is to use a stocking calculator that can take in your tank dimensions and fish species that you want and tell you if you are under or over stocked. It should calculate based on surface area and bioload.
Signs Your Tank Is Overstocked
If you find yourself doing frequent filter cleanings, water changes, or constantly fighting with algae blooms and cloudy water, you might be overstocked. You may also see increases in ammonia and nitrite, or even experience a complete cycle crash. Any of these symptoms could indicate overstocking.
If you are overstocked, that doesn’t mean you have to get rid of fish, try adding more live plants, feeding less, or even upgrading your filtration. If you do need to rehome fish, try to find somewhere locally, either another hobbyist or local fish store, to take them as adult fish are harder to ship than young fish.
