How many fish can I keep in a 20 gallon tank?

A 20 long with a decent hang-on-back can hold a tetra school of 10–12 plus six corydoras if you cycle first. Run the stocking planner with adult sizes — inch-per-gallon is not the math.

Is the inch-per-gallon rule accurate?

No. A 6-inch goldfish is not six neon tetras. This planner uses adult size, a waste factor, and filter turnover. Compatibility is a separate question — a tank can be understocked and still a fight.

Can I share my stocking plan?

Yes. Copy plan link encodes the tank, filter GPH, and species list in the URL. Nothing is uploaded.

Flagship tool

See the tank before you buy the bag

Pick a footprint. Add the fish at adult size. The glass, the bioload, and the fight flags update together. Copy a link when you want a second opinion. Nothing is uploaded.

Loading planner…

How to use this

Pick a standard tank or enter gallons. Stocking uses the box label. The glass shows 90% fill — that is the water you actually have.
Add species. The first click uses the minimum school. Fish in the glass are drawn at adult size against that footprint.
Read stocking % (waste vs water and filter), then the flags: school too small, tank too short, pair fights, goldfish in a 20.
Copy the plan link. Send it. Compatibility is still worth a second look on its own page if the mix is weird.

Bioload model

Water gallons = interior L × W × H × 0.9 fill
Bioload inches = qty × adult size × species waste factor
Capacity inches ≈ box gallons × (turnover / 4) clamped 0.6–1.5×
Stocking % = bioload / capacity × 100

Setups people actually buy

Percent is a model, not a permit. Load the example on the planner. A tank can be understocked and still a fight.

Plan	Tank	Call
1 betta	10 gal	Fine if filtered and heated
10 neons + 6 corys	20 long	Comfortable community start
6 fancy goldfish	20 gal	No — waste and adult size
1 common pleco	55 gal	Outgrows it; bristlenose instead
12 neons + 1 angelfish	29 gal	Risky once the angel is adult

Worth knowing

Add fish slowly over weeks so the filter colony can catch up.
Goldfish and big cichlids have high waste factors — volume alone is not enough.
Match filter upgrades to stocking growth; do not wait for ammonia spikes.

Usual mistakes

The old “inch per gallon” rule ignores body mass, waste, and filtration.
Stocking to 100% on day one of a new cycle.
Counting juveniles at adult size later — plan for full-grown fish now.