Decision rule
Hard conflicts stay visible
A non-overlapping reviewed temperature, pH or GH range produces an unsuitable result. A large size difference combined with predatory behaviour can do the same. Territory, fin risk, limited overlap and broad aggression flags normally produce caution. The checker never turns more filtration into proof that predation or social conflict is safe.
Reproducible examples
Current outputs from the live rules
Betta fish + Guppy
Compatible with cautionBetta fish and Guppy share some workable conditions, but behaviour, adult size, fin risk, territory or limited water overlap needs active planning.
Check this pair in the live tool →Betta fish + Fancy goldfish
Compatible with cautionBetta fish and Fancy goldfish share some workable conditions, but behaviour, adult size, fin risk, territory or limited water overlap needs active planning.
Check this pair in the live tool →Freshwater angelfish + Guppy
Usually unsuitableFreshwater angelfish and Guppy have at least one major water, adult-footprint or predation conflict to resolve before they are considered together.
Check this pair in the live tool →Limitations
What the rules cannot observe
The structured records cannot see the exact aquarium layout, sex, breeding state, health, prior social history or individual temperament. A favourable baseline therefore remains conditional. Introductions need observation and a safe separation plan.