How the stocking estimate works
The calculator starts from the well-known 1 cm of fish per litre rule and improves it in three ways. Each species uses its adult length. Deep-bodied or messy fish such as angelfish, discus, goldfish and oscars carry a bioload factor above 1, because a 15 cm discus produces far more waste than five 3 cm tetras. And the tank’s capacity is nudged up for strong filtration and heavy planting.
Overstocking shows up as poor water quality: ammonia, nitrite and nitrate rise faster than the filter and your water changes can remove them, and oxygen can run short, especially at night.
What the stocking levels mean
| Stocking | Level | What to expect | Weekly water change |
|---|---|---|---|
| Under 50% | Light | Very forgiving; room to add fish | 20–25% |
| 50–80% | Moderate | Comfortable for most community tanks | 20–25% |
| 80–110% | Full | Fine with a good filter and regular maintenance | 25–35% |
| 110–140% | Heavy | Needs strong filtration and frequent testing | 35–50% |
| Over 140% | Over | High risk of ammonia, nitrite and low oxygen | 35–50% and reduce fish |
Worked example: a 100 litre planted tank
Fish load: 10 harlequin rasboras × 4.5 cm = 45, 6 panda corydoras × 5 cm = 30, 6 otocinclus × 4.5 cm = 27, and 1 dwarf gourami × 9 cm = 9. All have a bioload factor of 1, so the total is 111 cm.
Capacity: 100 L × 1 (normal filter) × 1.1 (heavily planted) = 110. Stocking = 111 ÷ 110 = 101%, which is “full”. The calculator also checks that every species is above its minimum group size, that the tank meets each one’s minimum size (60 L here), and that they share a temperature range (22–25 °C) and pH range (6–7.5).
Why the percentage is not the whole story
- Minimum tank size beats the rule. Two fancy goldfish only score 40% in 150 L, but OATA recommends at least 100 L for one adult plus 50 L per extra fish, so 150 L is the real minimum.
- Schooling fish need a group. Neon tetras kept in threes are stressed even if the tank has room; the calculator warns below each species’ minimum group.
- Shared water conditions. Discus need 28–31 °C; bronze corydoras prefer 22–26 °C. No amount of space fixes that.
- Temperament and size. Adult angelfish eat neon tetras; tiger barbs nip long fins. See the compatibility checker for every pair.
- New tanks need time. Add fish gradually so the filter bacteria can catch up; track the cycle first.
