Stocking Density
"One litre per centimetre" only holds for small, slim fish. In the same 136 L, goldfish (heavy waste output) get a quota of just 39–52 cm while cherry shrimp can run past 300 — the difference is the fish, not the water. So this tool gives a **total adult-length quota** by fish type, and leaves the headcount to you: divide by how big that species actually grows.
Stocking is a total adult-length quota, not "one centimetre of fish per litre": a 900×450×450 tank has about 136 L of swimmable water, which supports roughly 97–131 cm of adult fish for a typical community.
Units
Input parameters
in
in
in
in
%
GPH
Filtration is currently judged on 180 GPH
days
Fish type
Result
- Swimmable water (substrate and hardscape deducted)
- 36 US gal
- Inner cavity volume
- 43.1 US gal
- Recommended filter flow
- 180 GPH
- Total adult-length quotaBased on 1.2 L of water per cm of adult length
- 97–131 cm
- Actual turnover
- 5.0×/h
- Whole setup weight (approx.)
- 410.9 lb
Fish-type coefficients
| Fish type | Water per cm | Quota for this tank |
|---|---|---|
| Small slim fish | 0.9 L/cm | 152 cm |
| Freshwater community | 1.2 L/cm | 114 cm |
| Goldfish / koi | 3 L/cm | 45 cm |
| Medium cichlids | 2.4 L/cm | 57 cm |
| Marine fish | 2 L/cm | 68 cm |
| Shrimp | 0.35 L/cm | 390 cm |
Notes
- Water is taken at 8.3 lb/US gal; substrate and hardscape only remove volume, they are not swimming space
- The quota is a **sum of adult lengths**, not a headcount: a fish that reaches 30 cm occupies its 30 cm from the day you buy it
- Filtration can only adjust the quota within 1.35× — no amount of flow beats dilution by water volume
- Territorial behaviour is not modelled: cichlids and bettas chase tank mates, so discount the result further for them
The calculator above follows the unit system selected at the top of this page; the explanatory text below stays metric (mm / L / kg / ℃).
Need a full design?
- Want to turn the dimensions into a 3D model and a complete bill of materials
- Enter length, width and height in the aquarium designer and it generates the 3D model, dimensions, glass cut list, silicone usage and equipment selection, all exportable as CSV and PNG.
How to use it
- Step 1Measure the tank and the substrate depthThe inner cavity is the outer size minus twice the glass thickness (0 means auto), and the swimmable volume then loses the substrate and the hardscape share.
- Step 2Enter the measured filter flowUse the **measured** flow, not the pump rating (catch the outlet in a jug for a minute); leave 0 to assume the recommended 5× the water volume per hour.
- Step 3Set the water change interval and pick a fish typeA longer interval than the type wants discounts the quota; the fish type sets how many litres of water each centimetre of adult length needs.
- Step 4Read the total adult-length quotaQuota ÷ the adult length of the fish you want gives the headcount. A 97–131 cm quota, for example, is about 19–26 tetras that reach 5 cm.
Frequently asked questions
- Is "one centimetre of fish per litre" right?
- It roughly holds for small, slim, moderately active fish. This tool breaks it into a fish-type coefficient: 0.9 L/cm for tetras, 1.2 for a community, 3.0 for goldfish, 2.4 for medium cichlids, 2.0 for marine fish and 0.35 for shrimp. In the same 136 L tank goldfish get 39–52 cm of quota while shrimp can pass 300.
- How many fish can a 90×45×45 tank hold?
- About 136 L of swimmable water (163 L inner cavity, less substrate and hardscape). A typical community quota is 97–131 cm of adult length: about 19–26 tetras that reach 5 cm, or 9–13 angelfish that reach 10 cm. Treating length as the budget works far better than counting heads.
- Does more filter flow mean more fish?
- Yes, up to a point. The tool turns filtration into a 0.6–1.35× factor: below 3× per hour the quota drops to 0.6, and past 12× per hour it only reaches 1.35. Filtration cannot beat dilution — nitrate accumulation is set by how much water you change, not by how fast you move it.
- Why count adult length instead of the fish I bought?
- Because a fish that reaches 30 cm occupies its 30 cm from the day you buy it at 3 cm. Upgrading the tank later is usually the real cause of a sudden water-quality crash: the filter has not changed, but the waste output has gone up tenfold.
- How does water change frequency affect stocking?
- Once the interval passes what the fish type wants (7 days for goldfish, 10 for cichlids, 14 for most freshwater fish, 21 for shrimp) the quota is discounted proportionally, down to 55%. It is not an outright "no", because changing less water means the same fish need more volume rather than becoming physically impossible.
- Can I stock cichlids or bettas to the full quota?
- No. This estimate covers the water-quality load only, not territorial behaviour. Cichlids, bettas and some bottom dwellers chase their own kind, so discount the result further — start around 60–70% of the quota and adjust after watching for fighting and hiding.