Overstock a tank and you find out the hard way, cloudy water, gasping fish, an ammonia spike at 2am. The aquarium stocking calculator gives you a real number before you buy a single fish, not after. Plug in your gallons, your fish size and bioload, and your filter, and it tells you where you actually stand.
Aquarium Stocking Calculator
What your number means
The calculator spits out a bioload-adjusted fish capacity, not just a gallon count. Here is what to do with it depending on where you land.
Understocked
You have room to grow. That is the safe side to be on, and it means your filter and water changes are carrying less load than they could handle. Add fish slowly, a few at a time, and let the tank cycle between additions. Browse the aquarium supplies guide for filtration that scales with you as you add fish.
Right
You are sitting close to the calculator’s number. That is the target, not a floor to push past. Keep your filter flow rate where the calculator recommends and hold your water change schedule steady, and this tank should stay stable for years.
Overstocked
You are past the number, and that is where ammonia and nitrite problems start. The fastest fix that does not involve rehoming fish is a stronger filter and tighter water change discipline. See stronger aquarium filters before you do anything else.
How this calculator works
The math starts with the old “inch per gallon” rule and corrects the two things it ignores, bioload and adult size. Your fish capacity is gallons divided by adult length in inches, then divided again by a bioload multiplier for how messy that species is. A planted tank gets a small bonus since live plants pull some nitrogen out of the water for you.
| Input | What it changes |
|---|---|
| Bioload class | Low (tetras, rasboras) x0.7, medium (livebearers, gouramis) x1, high (goldfish, cichlids, plecos) x1.6 |
| Filter flow | Recommended minimum is 5x your gallons, the calculator flags anything under 4x turnover |
| Live plants | Heavily planted tanks get roughly 15 percent more capacity |
| Weekly water change | 10 to 25 percent is the commonly recommended range |
| Tank shape | Standard rectangular gives the most surface area per gallon, tall or bowl-shaped tanks stock more conservatively |
The water change range and the surface area guidance follow the same principles Cornell Cooperative Extension publishes in its aquaculture and home aquarium water quality guides, gas exchange happens at the surface, so shape matters as much as volume.
Mistakes I made before I did the math
I do not keep fish. I keep a Mastiff, barn cats, two rescue rabbits and a desert tortoise. The mistakes below are the ones that come up again and again in the reviews and the extension guides I dug through building this calculator.
- Buying fish at the size they are in the store tank, not the size they will be as adults, then wondering why the tank looks crowded six months later.
- Undersizing the filter to save money, then fighting cloudy water and algae for the life of the tank.
- Skipping water changes because the tank “looks fine,” when ammonia and nitrite do not show up as cloudy water until the problem is already serious.
- Treating every fish the same size class, when a goldfish and a tetra of the same length put very different bioloads on the same gallon count.
- Ignoring tank shape, a tall narrow tank holds the same water as a wide one but offers less surface for gas exchange.
Share or embed this calculator
If you run a fish club forum, a rescue’s aquatics page, or a fishkeeping blog, this aquarium stocking calculator is free to embed with the credit line intact. Give your readers the same number before they buy fish instead of after.
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Aquarium stocking calculator FAQ
How many fish can I put in a 20 gallon tank?
It depends on adult size and bioload, not just gallons. A 20 gallon tank of 2 inch, medium bioload fish lands around 10 fish by this calculator, run your own numbers above since species varies the answer a lot.
Is the 1 inch per gallon rule still accurate?
It is a starting point, not the full picture. It ignores adult size, bioload, filtration and tank shape, which is why this calculator adjusts for all four instead of stopping at one ratio.
Do live plants really let me stock more fish?
Yes, a modest amount. A heavily planted tank gets roughly a 15 percent bump in this calculator’s estimate since plants take up some of the nitrogen waste your filter would otherwise carry alone.
How often should I change the water?
10 to 25 percent weekly is the commonly recommended range. Below that, waste builds up, above that is fine if your tap water and fish tolerate it.
Does tank shape actually matter for stocking?
It does. Standard rectangular tanks give the most surface area per gallon for gas exchange, tall or bowl-shaped tanks have less, so stock those more conservatively than the raw number suggests.
What filter flow rate do I need?
Aim for 5x your tank’s gallons per hour, and treat anything under 4x turnover as a sign to size up. See stronger aquarium filters if your current filter falls short, or heaters sized for bigger tanks once you are past 40 gallons.
Gunner Vance