How to use a stocking calculator

What a stocking calculator does

A stocking calculator lets you pick a tank size (in litres or gallons), add species, and see whether your planned combination fits. It checks bioload, compatibility between species, group sizes, and tank minimums. The goal: avoid cramming too many fish into too little water, or mixing species that will fight or stress each other. Tools like our free stocking calculator on the site, or a fish tank app such as App-aquatic, give you a quick sanity check before you head to the shop.

What it can’t replace

Calculators use rules and data, not your actual tank. They don’t know your filtration strength, how often you change water, or whether your tap is hard or soft. They won’t catch every personality clash or every species that needs a specific setup. Treat them as a planning aid, not a guarantee. Always research each fish and why compatibility matters.

How to use one effectively

• Start with tank size. Enter your real volume — litres or US gallons — so bioload estimates make sense.
• Add species in the order you plan to stock. Some calculators show compatibility as you go; others give a summary at the end.
• Respect group sizes. Schooling fish need groups of six or more; a calculator will flag if you add too few.
• Watch the compatibility matrix. Red or amber warnings mean rethink that pairing.
• Leave headroom. If the tool says you’re at 90% bioload, consider stopping there. See overstocking for why.

Litres vs gallons

UK and European tanks are often sold in litres; US tanks in gallons. A 100 litre tank is roughly 26 US gallons. Good calculators support both — pick the unit that matches your tank label. Our calculator includes popular models (JUWEL, Fluval, Aqueon, etc.) so you can select by brand and size.

Quick takeaways

• Stocking calculators help you plan combinations and spot obvious problems before you buy.
• They don’t replace research or knowledge of your own setup.
• Use them with bioload sense and compatibility in mind.