Do the math before you stock
AquaMath is a set of aquarium calculators for fishkeepers who want the arithmetic shown, not hidden. Tank volume in litres and gallons, stocking density by surface area, water-change nitrate math, substrate weight and heater wattage — every rule traced to a named source.
The calculators
Pure arithmetic, no accounts, nothing stored — and the methodology is printed under every tool.
Tank volume & unit converter
Rectangle or cylinder dimensions to litres, US gallons and UK gallons, with glass-thickness displacement.
CalculatorStocking density (surface area)
How many fish fit? Every published surface-area rule computed side by side, plus the 6× length check.
CalculatorWater change & nitrate
Exact dilution math: what one, two or ten changes of any size actually do to your nitrate reading.
CalculatorSubstrate amount
Tank floor area × desired depth = litres of sand, gravel or aquasoil, converted to kilograms and pounds.
Guides
The concepts behind the calculators — sizes and rules from named, verifiable aquarium sources. Last verified: 2026-11-16.
The nitrogen cycle, in plain numbers
Ammonia to nitrite to nitrate: what the bacteria actually do, and why an uncycled tank kills fish.
SpeciesHow big a tank? Neon tetras, bettas, goldfish
The three classic mis-stocked fish, with adult sizes and tank recommendations from care-guide sources.
StockingWhy "1 inch per gallon" fails
Surface area, bioload, adult size and schooling: the modern critique of the oldest stocking rule.
MaintenanceWater-change math explained
Two 50% changes leave 25% of the original nitrate — percent removal is not additive. Here is the algebra.
SetupCycling a new tank: methods compared
Fishless with ammonia, ghost-feeding, bottled bacteria, fish-in — trade-offs of each, with timelines.
EquipmentHeater sizing for your room
Watts per gallon is only half the story — the gap between room temperature and target is the other half.
SetupSubstrate depth: planted vs bare
How deep for roots, how deep is too deep, and why bare bottom suits some tanks better.
StockingStocking mistakes that kill fish
Overcrowding, incompatible aggression, adding fish too fast — practical framing, not fishkeeper folklore.
HealthQuarantine tank basics
A spare 10 gallons and five weeks of patience is the cheapest insurance in the hobby.
Browse by topic
Jump straight to what you are planning.