Aquarium Size Calculator: How Many Fish Fit?

This direction of the aquarium math answers the shopping question: given the tank you already own, how much stocking can it hold? The calculator takes the actual water volume — nominal litres minus 10-15% displacement from substrate and hardscape — then applies the classic 1 cm of adult fish per litre guideline with a further 20-25% margin for waste load and growth. The result is a budget of adult fish centimetres you can spend species by species. Worked example: a 120 L tank holding about 105 L of real water gives a capacity near 84 cm of fish — a school of 12 neon tetras (48 cm) plus 6 corydoras (30 cm) fits with headroom, but adding a 15 cm pleco takes it to the limit.

1. 1

   Step 1

   Enter your tank's nominal volume or its internal dimensions

2. 2

   Step 2

   Set the displacement percentage for substrate and decor

3. 3

   Step 3

   Read your capacity budget in cm of adult fish

4. 4

   Step 4

   List planned species by adult length and compare totals

• 1

  Count adult size, never purchase size

• 2

  Test nitrate monthly: rising nitrate means over-budget

• 3

  Bottom dwellers need floor area, not just litres

• 4

  New tanks: stock to 50% for the first six weeks

• Counting the tank's label litres as real water

• Stocking fully on day one of a new filter

• Mixing incompatible territories inside the budget

• Forgetting that schools count as multiple fish