Aquarium Stocking Calculator
The inch-per-gallon guideline
A simplified stocking model divides the usable water volume by the average adult fish length to estimate a headcount . While crude, the rule-of-thumb keeps new aquarists from wildly overstocking. Experienced keepers adjust the result downward for bulky species and upward for schooling nano fish.
The calculator rounds down to the nearest whole fish to reflect practical limitations. Always monitor water quality and adjust stocking density based on filtration and behavior.
Worked example
For a 40-gallon breeder tank filled with schooling tetras averaging 1.5 inches, the guideline yields 26 fish. If your filtration is moderate and you want to leave margin for growth, you might stock 20–22 individuals instead. Use the copy button to record your plan alongside equipment notes.
Scenario comparisons
Review how stocking recommendations vary for different aquarium goals.
| Tank type | Volume | Average length | Guideline count |
|---|---|---|---|
| 20 gal community | 20 gal | 2 in | 10 fish |
| 55 gal cichlid tank | 55 gal | 4 in | 13 fish |
| 75 gal planted nano | 75 gal | 1 in | 75 fish |
Beyond the formula
Filtration capacity, aquascape layout, and species temperament all influence real-world limits. Heavy feeders like goldfish or cichlids may require half the guideline density, while densely planted shrimp tanks can exceed it safely. Use water testing to validate your plan and adjust gradually.
Expand your planning toolkit with the Aquarium Filter Size Calculator, Water Change Schedule Calculator, and Aquarium CO₂ Injection Calculator to balance nutrients, oxygen, and filtration for the fish count you select.