Methodology
How Fishori decides.
Fishori checks whether two fish are realistically suited to the same aquarium. It looks at the things that usually make or break a pairing, and it errs on the side of caution. Where sources disagree on a number, we take the stricter one.
What we check
1. Predation and physical risk
Could one fish realistically eat or injure the other? We compare adult sizes, not the size they are in the shop. A fish that’s known to eat smaller tank mates, or that hunts at the surface, gets less benefit of the doubt.
Armoured bottom-dwellers like corydoras and plecos are a special case. Most fish won’t eat them, but might harass them, so we flag that as a caution rather than a no. A large true predator is still a no. Shrimp and snails get their own check, because plenty of fish that never bother another fish will happily clear out a shrimp colony.
2. Temperament and behaviour
Peaceful, semi-aggressive and aggressive fish don’t mix freely. An aggressive fish with a peaceful one is a serious problem. One step apart can work with space and cover, so we flag it as a caution.
Fin nippers are flagged against long-finned fish. Some species only nip when they’re kept in too small a group or too small a tank, and we treat those more leniently than a habitual nipper.
3. Water requirements
Both fish need to be comfortable in the same water. We check temperature and pH. If the ranges don’t overlap at all, that’s a serious problem. If they only just overlap, it’s a caution, because there’s no margin for a hot summer or a drifting pH. We also flag a soft-water specialist paired with a hard-water one, and freshwater fish paired with brackish species.
4. Tank size and group needs
Tell us your tank size and we check it against the larger of the two published minimums. Schooling fish need proper numbers, and a lone schooling fish is a stressed one, so the pair checker reminds you of minimum group sizes.
The tank builder goes further. It knows how many of each fish you’re planning, so it checks group sizes, overall stocking and the shared water window for the whole tank, not just two fish at a time.
5. Known exceptions
General rules are sometimes too crude. For a small number of well-documented combinations, we adjust the result to match what experienced keepers consistently see. That cuts both ways: some pairings that pass on paper are marked risky because they’re known to go wrong, and a few that trip a blunt rule are softened to a caution because the combination is routinely kept successfully. None of these adjustments turn a predation risk into a pass.
6. Confidence and limits
Source quality varies. Every fish page shows how well its numbers agree across sources. Where a fish has thin sources, or is often mislabelled in the trade, a pairing that would otherwise be GOOD is shown as CAUTION instead.
Fish are individuals, and we can’t see your tank. Treat every verdict as conservative planning help, not a guarantee.
How the verdict works
Every check runs. The verdict comes from the whole picture, not the first problem we hit.
- GOOD
- Generally compatible in an appropriate setup.
- CAUTION
- Can work, but there’s something important to plan around.
- RISKY
- There’s a significant reason not to recommend the pairing for a typical setup.
The pair page leads with the most important issue and lists the rest underneath, each with its reasoning. Nothing is hidden behind a score.
A worked example
This example is generated from the live engine each time the page is built, so it always matches the pair page.
Betta + Neon Tetra
Verdict: CAUTION
Why:
- Their temperaments can clash — betta is semi-aggressive, neon tetra is peaceful.
- Their preferred temperatures only overlap in a narrow band (24–26°C).
- Neither of them preys on other fish, so predation isn't the concern here.
- Neon Tetra still need a proper group — at least 6.
This pairing can work in the right tank, but it isn't an automatic yes.
Profile confidence: Betta high · Neon Tetra high
What we don’t do
- Predict the personality of your individual fish. Nothing does.
- Model cycling, nitrate or filter maturity. Those depend on your setup, not the species pairing.
- Recommend specific products, brands or shops.
- Replace your own eyes. If your fish are stressed and our verdict says GOOD, trust the fish.
The full list is on the limitations page.
Corrections
We review profiles and verdicts when the evidence changes, and we fix them when we’re wrong. If you spot a bad number, a dodgy verdict or a mixed-up species, email hello@fishori.com. We read every report.