Aquarium Stocking Limit Calculator
Fish tank stocking: enter tank size, filter and the fish you plan to keep to see their bio-load in inches against the tank limit, from Good to Overstocked. How it works
Use the Aquarium Stocking Limit Calculator
About the Aquarium Stocking Limit Calculator
A 20-gallon tank on a basic filter has a stocking limit of 16 inches of fish here: one inch per gallon, times 0.8 for the filter. Six neon tetras at 1.5 inches use 9 inches, 56.3% of it, which reads Good. The fish tank stocking calculator does not give a fish count; you build the list, and it adds up each fish’s adult length, weighted for how messy the species is, against that limit.
Stocking limit in inches of fish by tank size and filter (rating 100%)
| Tank | Basic (0.8×) | Hang-on-back (1.0×) | Canister (1.5×) | Sump (1.8×) |
|---|---|---|---|---|
| 10 gal | 8.0 in | 10.0 in | 15.0 in | 18.0 in |
| 20 gal | 16.0 in | 20.0 in | 30.0 in | 36.0 in |
| 29 gal | 23.2 in | 29.0 in | 43.5 in | 52.2 in |
| 40 gal | 32.0 in | 40.0 in | 60.0 in | 72.0 in |
| 55 gal | 44.0 in | 55.0 in | 82.5 in | 99.0 in |
| 75 gal | 60.0 in | 75.0 in | 112.5 in | 135.0 in |
The limit is gallons × filter factor × rating ÷ 100. A Good reading needs the weighted load at or under 60% of the figure in the table.
How to use this tool
- Enter the tank volume in gallons or litres and pick the filter type.
- Set the filtration rating: 100% for a filter rated for this tank, less for an undersized or clogged one, more for one rated for a bigger tank.
- Choose the water type, then add each species with its adult length and how many you plan to keep.
- Optional: enter the water surface length and width for a surface limit, and the filter’s rated flow in gallons per hour for its turnover.
- Read the bar: up to 60% of the limit is Good, up to 80% Monitor, up to 100% Caution, and above that Overstocked. Check the school-size, territorial and compatibility warnings under it.
Frequently asked questions
- How many neon tetras can go in a 20 gallon tank?
- On a basic filter the limit is 16 inches and each 1.5-inch neon counts as 1.5. Six read Good at 56.3%, eight read Monitor at 75%, ten read Caution at 93.8% and eleven are Overstocked at 103.1%. On a hang-on-back filter the limit is 20 inches, so eight neons drop to 60%, still Good.
- Is the one inch per gallon rule right?
- It is a rough freshwater rule of thumb. It ignores body shape, so a heavy-bodied fish loads the filter more than a slim one of the same length, and it ignores the tank’s shape. The calculator weights species by a factor from 1.0 to 2.0 and can add a surface limit to make up for some of that, but it cannot judge swimming room.
- Does the water surface area matter?
- Enter the surface length and width and the page adds a second limit of 1 inch of fish per 12 square inches of open surface, a hobbyist rule of thumb from a retailer’s guide, not a measured figure. It counts plain inches, and when it is stricter than the volume limit it sets the percentage. Twelve 1.5-inch neon tetras in 20 gallons on a canister filter read 60% by volume; a 24 × 12 inch surface supports 24 inches, so they read 75%, Monitor.
- Does a canister filter let me keep more fish?
- In this calculator, yes: a canister multiplies the limit by 1.5 and a sump by 1.8, against 0.8 for a basic filter. The same 20 gallons gives 16 inches on a basic filter and 30 on a canister. Better filtration does not add swimming space, so a large fish still needs a large tank.
- Why does one betta push my tank over the limit?
- A betta has a species factor of 2.0 and, as a territorial fish, another 50% on top, so a 2.5-inch betta counts as 7.5 inches. Added to six neon tetras in 20 gallons on a basic filter it takes the load from 9 to 16.5 inches, 103.1%, and also raises a compatibility warning with the tetras.
- Does it work for saltwater tanks?
- It lists five saltwater and two brackish species, but it uses the same one-inch-per-gallon base for every water type. Marine stocking is harder to judge from volume alone, so treat a saltwater reading as a loose guide and check each species’ own tank size needs.
Related Tools
View all toolsFish Tank Nitrogen Cycle Calculator
Enter ammonia, nitrite, nitrate, pH and temperature to see your tank's cycle phase, the free (toxic) ammonia share and a water change suggestion.
EtG Calculator
Hours since your last drink and a cutoff of 100, 250, 500 or 1,000 ng/mL give published EtG urine detection rates. It does not predict a result.
Peptide Calculator
Enter the peptide in the vial (mg), the bacteriostatic water added (mL) and the dose (mcg) to get the units to draw on a U-100 insulin syringe.
Pore Clogging Checker
Paste a skincare ingredient list to find comedogenic, pore-clogging ingredients graded 3 to 5 in Fulton's 1989 rabbit-ear tests, with each grade.
Blood Pressure Category Checker
Blood pressure category for one adult reading on the American Heart Association chart. Systolic and diastolic are rated separately; the higher one counts.
Dog Pregnancy Calculator
Dog due date 63 days after mating (day 56 to day 70) or after a progesterone-timed ovulation (day 61 to day 65), from Cornell veterinary figures.