Pets
What size aquarium do you need (litres and heater)?
Net liters of your tank from its dimensions, heater wattage needed and how many fish it can host.
Quick answer
Every aquarium decision downstream — filter, heater, lighting, how many fish — hangs on a number that isn’t the one printed on the box: net liters. Multiply the tank’s dimensions and you get gross volume, but gravel, rocks, wood and the few centimeters you leave below the rim eat about 15% of it. An "80-liter" tank holds roughly 68 liters of actual water, and dosing medication or fertilizer by the label number instead of the real one is a classic beginner’s overdose.
Net water volume
107 L
- Gross volume
- 126 L
- Heater wattage
- ~125 W
- Fish budget (small community fish)
- ~54 cm of fish
Net volume assumes ~85% of gross: substrate, rocks, wood and the fill line eat the rest. Heater rule: ~1 W per net liter in a heated room, 1.5 W/L in cold rooms. The fish line uses the conservative 1 cm of adult fish per 2 net liters, valid for small community species (tetras, rasboras, guppies) — territorial or large fish need species-specific research. Cycle the tank 3-4 weeks before the first fish.
How it works
From net liters, the sizing rules are refreshingly simple. Heater: about 1 watt per liter in a normally heated room, 1.5 W/L in cold rooms — rounded up to the standard sizes (50, 100, 150, 200 W). Stocking: the calculator uses the conservative 1 cm of adult fish per 2 net liters, appropriate for small community species like tetras, rasboras and guppies; remember to count adult sizes, because the 3 cm baby pleco at the shop becomes a 30 cm tank-buster. And the rule that trumps all others: a new tank must cycle 3-4 weeks before the first fish, letting nitrifying bacteria colonize the filter — the single step whose skipping kills more fish than every other mistake combined. A water test kit is how you know the tank is actually ready.
Frequently asked questions
How many fish can I keep in a 60-liter tank?+
A 60 L gross tank holds ~50 net liters, which budgets about 25 cm of small adult fish — for example a school of 10 neon tetras (4 cm each would exceed it, so realistically 8-10 neons and little else, or 6 guppies plus some shrimp). Schooling species need groups of 6+ to behave naturally, so in small tanks choose one species and do it well rather than a zoo of pairs.
What heater wattage does my aquarium need?+
One watt per net liter in a room that stays 18-22 °C, one and a half in colder rooms — a 100 L tank wants a 100-150 W heater. Two smaller heaters at the tank’s ends beat one big one: redundancy against the classic stuck-on failure, and more even temperature. Always pair with a separate thermometer; heater thermostats drift with age.
What is cycling and why does it take weeks?+
Fish waste produces ammonia, which is lethal even in traces. Two families of bacteria convert it — ammonia to nitrite (also toxic), nitrite to relatively harmless nitrate — and they need 3-4 weeks to colonize the filter in useful numbers. Add fish on day one and they swim in rising poison. Cycle with a bacteria starter and a pinch of fish food as ammonia source, test the water, and stock only when ammonia and nitrite read zero.
How often should I change the water?+
The standard is 20-30% weekly or biweekly, with a gravel vacuum to pull debris. Never change all the water or wash the filter media under tap water — chlorine kills the bacterial colony you spent weeks growing; rinse media in the removed tank water instead. Rising nitrates between changes (test kit) tell you if your schedule is keeping up with your stocking.
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