A 29 gallon tank with 8 neon tetras, 6 panda corydoras and 4 honey gouramis is 73% stocked: comfortable, with room for a few more. The old rule of one inch of fish per gallon calls the same tank 103% and says it is full. Both are rough, but the one that looks at adult size and waste is closer to what your filter actually has to process.
How the stocking level is calculated
The calculator gives every species a waste figure and every tank a capacity, then divides. A fish's waste units are its adult length in inches, squared, times a class factor: 0.8 for light eaters, 1 for typical community fish, 1.5 for large or messy fish such as goldfish, big cichlids and tangs. A tank holds 2 units per gallon with a standard filter, 0.75 times that with light filtration and 1.35 times with heavy.
Stocking % = Σ (count × adult inches² × class factor) ÷ (gallons × 2 × filter factor)
Length is squared because waste grows faster than length. Fish weight rises roughly with the cube of length, and resting metabolic rate rises with body mass to about the 0.8 power: Clarke and Johnston found a mean exponent of 0.79 across 69 species of bony fish, which works out near length to the power 2.4. We use 2, calibrated so that a minimum group sitting in the minimum tank the sources recommend lands at a median of 60% across the 115 freshwater species. The class factors and the factor of 2 are our editorial choices, not measurements, and the results are a guide, not a guarantee.
Worked example: a 29 gallon community
Neon tetras reach 3 cm (1.2 in): 1.2² × 0.8 = 1.1 units each, 8.9 for eight. Panda corydoras reach 5 cm (2.0 in): 2.0² × 0.8 = 3.1 each, 18.6 for six. Honey gouramis reach 5.5 cm (2.2 in): 3.75 each, 15.0 for four. That is 42.5 units against a capacity of 29 × 2 = 58, so 73%. On light filtration the same fish are 98%; on a heavy canister they are 54%. The calculator says four more panda corydoras or four more honey gouramis fit, or 13 more neons. All three species share 22 to 25 °C (72 to 77 °F) and pH 6 to 7.4.
Why one inch per gallon fails
Adding up inches treats a 10 inch fish as ten 1 inch fish. It is not: the big fish makes far more waste and needs swimming room. It also ignores body shape and diet. By the inch rule, the 29 gallon example is over by 3%, while a lone oscar (35 cm, 14 in) needs 14 gallons on paper and in practice a source-recommended 540 L (143 gal). The model above gives the oscar 285 units, or 143 gallons of capacity: the same figure the source gives, because the class factor for large messy predators is high.
Freshwater and saltwater
The species list covers 115 freshwater and 25 saltwater fish, and each one carries a source. Freshwater profiles come mostly from SeriouslyFish, with Aquarium Co-Op for a few, and the marine profiles from LiveAquaria. A species is hidden if the numbers the maths needs (adult size, minimum tank, temperature, temperament) could not be verified. Saltwater tanks are usually stocked lighter than freshwater and the waste model was calibrated on freshwater sizes, so on the saltwater tab the source's minimum tank size is the limit to respect first.
Levels and what they mean
| Level | Stocking | What to expect |
|---|---|---|
| Lightly stocked | up to 50% | Room to add, forgiving of missed water changes |
| Comfortably stocked | 51 to 90% | A normal healthy tank on weekly water changes |
| Fully stocked | 91 to 100% | Needs steady maintenance and testing |
| Overstocked | above 100% | Ammonia and oxygen problems get likely |
Capacity: what the checks add
The percentage covers waste only. Beside it the tool checks your tank against each species' minimum tank size, whether a schooling fish is below the group size the source recommends, species listed as one per tank, whether the fish share a temperature, pH and hardness range, and whether an aggressive fish sits next to peaceful ones or one that is three times the length of a small tankmate. Fix those before the percentage.
How many fish can I add
Use the "Room for more" figure per species, then add a few at a time. A tank still cycling can carry far less than its capacity: add fish gradually and test ammonia and nitrite. Fish grow: the calculator uses adult size, so a tank that looks empty with juveniles is the target, not the starting point.
Next steps
Know the true water volume first: the aquarium volume calculator subtracts substrate and rock, and the fish tank calculator adds the weight of the filled tank. Plan the bottom with the aquarium substrate calculator, and for a marine tank mix your water with the salinity calculator. Every aquarium tool is on the aquarium hub.