Stocking levels are based on either water volume or surface area. There are many variables including the size of fish & filtration system.
These are generally accepted stocking levels.
Cold water 1” of fish per gallon (2.5cm per 4.5 litres)
Tropical 1” of fish per gallon (2.5cm per 4.5 litres) in the first three months, 2” of fish per gallon (5cm per 4.5 litres) after the aquarium has been established for at least six months.
Pond 10” of fish per 100 gallons (25cm per 45 litres)
Marine fish only 1” of fish per 4 gallons (2.5cm per 18 litres) in the first six months, increasing to a maximum of 1” of fish per 2 gallons (5cm per 9 litres) after six months.
Reef aquarium 1” of fish per 4 gallons (2.5cm per 18 litres)
PLEASE NOTE stocking levels are for guidance only. Larger fish produce far more waste than smaller fish. Therefore, when keeping large fish stocking densities should be far lower.
All stocking levels assume a good filtration system has been operating for at least 3 months. The stocking level in new systems should be much lower.