Calculate your aquarium’s volume in gallons and liters, then check whether your planned fish stock is appropriate. This calculator uses bioload-based scoring, not the flawed inch-per-gallon rule.
📐 Tank Volume Calculator
Enter dimensions above to calculate.
🐟 Stocking Level Checker
Add your fish below. The calculator scores bioload relative to your tank size. Bioload varies by species — goldfish produce ~5× more waste than most tropical fish.
Auto-filled from volume calculator above
Add fish and click Check.
Reference: Popular Fish Bioload & Tank Requirements
| Fish | Min Tank (gal) | Bioload Score | Group Size | Notes |
|---|
Why the Inch-Per-Gallon Rule Fails
The “1 inch of fish per gallon” rule has been repeated so long it seems authoritative — but it’s fundamentally flawed in several ways:
- Ignores body shape: A 6-inch oscar has the body mass of dozens of 1-inch tetras. Volume matters, not just length.
- Ignores waste production: Goldfish and cichlids produce dramatically more ammonia per inch than small schooling fish.
- Ignores oxygen requirements: Active fish like danios need more dissolved oxygen than slow-moving bettas.
- Ignores surface area: Gas exchange happens at the water surface — tank shape matters as much as volume.
- Ignores filtration quality: A tank with excellent filtration can support more bioload than one with minimal filtration.
Our bioload scoring system is still a rough estimate — the real answer involves testing your water parameters regularly and adjusting accordingly. Fish load should be built gradually to avoid cycling crashes.
Common Tank Sizes and Stocking Guidelines
| Tank Size | Dimensions (typical) | Good For | Notes |
|---|---|---|---|
| 5 gallon | 16"×8"×10" | 1 betta, small shrimp colony | Very small margin for error; not for beginners |
| 10 gallon | 20"×10"×12" | 1 betta + snails, small nano community | Beginner-possible but limiting |
| 20 gallon long | 30"×12"×12" | Small community (tetras, corydoras, 1 centerpiece) | Best beginner size — long gives floor space |
| 29 gallon | 30"×12"×18" | Medium community, pair of dwarf cichlids | Popular compromise size |
| 40 gallon breeder | 36"×18"×16" | Medium-large community, breeding projects | Excellent floor area for bottom-dwelling fish |
| 55 gallon | 48"×13"×21" | Large community, medium cichlids | Narrow width limits some species |
| 75 gallon | 48"×18"×21" | Medium-large cichlids, large communities | First large tank most hobbyists get |
| 125 gallon | 72"×18"×22" | Aggressive fish, large cichlid pairs, predatory fish | Requires significant filtration investment |