Plan a healthy freshwater aquarium
Aquastock helps you stock the right fish for your tank. Use the calculator, check compatibility between any two species, and read care guides for 41 popular freshwater fish.
Open the stocking calculator →
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Popular categories
Fish families
View all 41 species in a comparison table →
Latest guides
Featured species
How to use this site
Three questions cover most of what people need before buying fish, and each has a tool or page here:
| Your question | Where to go |
|---|---|
| How many fish fit in my tank? | Stocking calculator |
| Can these two species live together? | Compatibility checker — 630 pairings assessed |
| What does this species actually need? | 41 full care sheets |
| My tank is X gallons — what can I keep? | Tank size guides, 5 to 125 gallons |
| How do I start a tank from nothing? | Beginner setup guide |
Why our numbers differ from other sites
Most minimum tank sizes online are inherited from decades-old convention. Ours are derived from adult size, swimming style, social group requirements and bioload, using a method we publish openly on the methodology page. That means you can check the reasoning rather than trusting the number — and it is why we sometimes recommend a larger tank than the shop does.
The other difference is that we treat group size as part of tank size. A species that needs a shoal of eight does not fit in the tank that holds one of them, and pretending otherwise is how most understocked, stressed community tanks begin.
Common questions
How many fish can I put in my tank?
It depends on adult size, group requirements, swimming zone and your maintenance routine — not on the old one-inch-per-gallon rule, which understates the waste produced by larger fish by a wide margin. Use the stocking calculator for a specific answer.
How long before I can add fish to a new tank?
Three to six weeks. The tank needs to complete a nitrogen cycle so that bacteria can process ammonia before fish produce it. See how to cycle a fish tank.
What is the best tank size for a beginner?
A 20-gallon long. Larger volumes are more forgiving of mistakes because waste and temperature swings are diluted, and the wide footprint supports a proper community.
Which fish are easiest to keep?
Harlequin rasboras, zebra danios, platies, cherry barbs and bronze corydoras are all hardy, peaceful and tolerant of a range of water conditions. See best fish for beginners.