Fish Tank Stocking Calculator
Enter your tank volume in litres, the adult size of the fish you want, and the stocking type. The calculator applies the centimetre-per-litre rule with the right multiplier, and sizes the weekly water change that keeps the chemistry stable.
Fish-shop advice counts the juveniles in the bags. This calculator counts the adults they become - the difference between a thriving tank and the silent ammonia crash that ends most first aquariums.
How to use
- Use the real water volume (tanks hold less than the label once gravel and filters move in - knock off 10%).
- Look up the ADULT size of your species, not the shop size, and enter it in cm.
- Pick the stocking type - community, territorial cichlids or waste-heavy goldfish - and cycle the tank before the first fish ever goes in.
Frequently asked questions
What is the 1 cm per litre rule?
A rough adult-size budget: total body length of adult fish per litre of water. It works for slim-bodied tropical communities; goldfish need 2.5 L per cm because their waste outruns their size, and territorial cichlids need extra room for behaviour, not just chemistry.
Why does adult size matter so much?
Shops sell juveniles, and most aquarium fish grow several times their purchase size within a couple of years. A tank stocked at shop size is overstocked by the time the fish mature - which shows up as persistent ammonia, torn fins and algae.
How long should a tank cycle before adding fish?
Four to six weeks without fish: the beneficial bacteria that convert ammonia live on filter media and take weeks to colonise. Fish-in cycling is possible with daily testing and changes, but fishless cycling with a drop of pure ammonia is kinder and surer.
How much water should I change weekly?
About 25%, dechlorinated and temperature-matched, vacuuming the gravel as you go. Consistency beats volume: a steady quarter weekly keeps nitrates under control without destabilising the chemistry the fish have adapted to.