About Fish That Fit
Fish That Fit is a free aquarium stocking calculator. You enter your tank size and the fish you keep or want, and it returns a bioload percentage, plain-English compatibility warnings, and a list of fish that would still fit, for both freshwater and saltwater tanks. Everything runs in your browser; there is no account, no paywall, and nothing to install.
Why we built it
“How many fish can I put in my tank?” is one of the most common questions in the hobby, and the honest answer, “it depends,” is hard to act on. The advice online is either an over-simplified inch-per-gallon rule or buried in forum threads. We wanted a single page that does the maths and the compatibility check at once, so a beginner can make a safe decision in under a minute and an experienced keeper can sanity-check a new combination.
How the bioload calculation works
Instead of the inch-per-gallon rule, every species in our list carries a stocking weight based on its adult size and typical waste output. A messy 12-inch oscar counts for far more than its length suggests, while a slim 2-inch tetra counts for less. We total the weighted load of your fish, scale your tank’s capacity by its filtration, and express the result as a percentage. We treat roughly 85% as a comfortable ceiling for most community tanks, leaving headroom for feeding, growth, and the occasional missed water change.
How the compatibility checks work
Each fish record stores its temperament, schooling needs, swimming level, adult size, and preferred temperature and pH. Fish That Fit compares your selections and flags the classic conflicts: aggressive fish with peaceful ones, fin-nippers with long-finned tankmates, shoaling fish kept in groups that are too small, predators that can swallow their neighbours, and species that want different water than the rest of the tank.
Where the data comes from
The species data is curated by hand from widely accepted hobby references and our own fishkeeping experience. It is a static dataset baked into the site, and there is no live database to drift out of date, which is exactly why the tool stays fast and reliable. Fish are individuals, though, and tank size, aquascaping, and stocking order all change outcomes, so treat every result as well-informed guidance rather than a guarantee.
An important caveat
No calculator replaces observation. Watch your fish, test your water, and be ready to adapt. If you spot an error in our data or have a species you’d like added, email hello@fishthatfit.com. We read everything.