Fish Stocking Calculator
Estimate a safe stocking level for your tank to avoid overcrowding.
How it works
A common guide is 1 cm of adult fish length per litre is far too high; a safer modern guideline is roughly 1 cm of slim-bodied fish per 2 litres of actual water, adjusted down for messy or large-bodied fish and up for heavy filtration and live plants.
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Formula & Method
A common guide is 1 cm of adult fish length per litre is far too high; a safer modern guideline is roughly 1 cm of slim-bodied fish per 2 litres of actual water, adjusted down for messy or large-bodied fish and up for heavy filtration and live plants.
Example Calculation
A 100 L planted tank with good filtration could hold ~12-15 small tetras (≈3 cm each) plus a small clean-up crew, leaving a safety margin for water quality.
Frequently Asked Questions
Is the 1 inch per gallon rule reliable?
No. It ignores body mass, waste output, swimming needs and territoriality. Waste scales with the cube of body length, so a 20 cm fish produces roughly eight times the load of a 10 cm fish, not twice. Use it only as a rough ceiling and stock well below it.
How many fish can I put in a 4ft tank?
A standard 243-litre four-foot tank with good filtration comfortably holds around 25-30 small community fish of 4-6 cm, or a much smaller number of larger fish. Planting, turnover rate and water change frequency all shift the figure.
Does live planting increase stocking capacity?
Modestly. Plants consume nitrate and some ammonium directly, which helps, but they do not replace biological filtration. Treat heavy planting as worth perhaps a 10-15% increase, not a doubling.
How fast should I add fish to a new tank?
Add a small group, wait two weeks, test ammonia and nitrite, then add the next group. Filling a newly cycled tank in one purchase overwhelms the bacterial colony and causes a secondary ammonia spike.
Does a sump increase how much I can stock?
Yes, both by adding water volume and by holding more filter media. Include the sump's operating water volume in your total when calculating.