Fish Tank Stocking Calculator
Calculate how many fish your aquarium can safely hold based on tank size and fish sizes.
Uses the inch-per-gallon rule with adjustments.
Fish tank stocking determines how many fish can safely live in an aquarium without compromising water quality, oxygen levels, or the fish’s health. Overstocking is the #1 cause of fish deaths in home aquaria.
The Classic Rule (Surface Area Method):
Max fish load = Tank surface area (cm²) / 12
This gives the total length of fish (in cm) the tank can support. For inches: surface area (in²) / 1 gives total inches of fish.
The Better Formula (Bioload Method):
Effective capacity (liters) = Tank volume × Filtration efficiency factor
- Understocked, good filtration: factor = 1.2
- Standard setup: factor = 1.0
- Overpowered sump/canister: factor = 1.5
Then: Total fish inches = Effective capacity (liters) × 1.0 (tropical freshwater)
Worked Example:
- 100-liter tank with a good canister filter (factor = 1.5)
- Effective capacity = 150 liters
- Max total fish length = 150 cm (about 60 inches)
- Planning to keep 10 neon tetras (each 4 cm = 40 cm total) + 2 corydoras (each 6 cm = 12 cm) + 1 angelfish (15 cm)
- Total = 67 cm → well within limit
Species-Specific Rules:
- Goldfish produce 3–4× the waste of tropical fish — use a stricter 1 cm per 5 liters rule
- Saltwater fish need double the space of freshwater equivalents
- Bottom-dwellers compete for floor space, not water column
- Territorial species (cichlids, bettas) need space calculations beyond simple length
Practical Tips:
- Add fish gradually over weeks to allow the biological filter to establish
- Always cycle a new tank for 4–6 weeks before adding fish
- Test ammonia and nitrite weekly — they spike before fish show visible stress