Use species-specific markers
Gonopodia in livebearers, fin shapes, body depth, breeding tubes, colour and behaviour may help, but no generic rule covers all tropical fish.
Choose health over extremes
Select active fish with normal swimming, appetite and body shape. Avoid breeding deformities, chronic buoyancy problems or fish unable to feed normally.
Allow compatibility to emerge
Two fish of opposite sex are not automatically a pair. Cichlids and bettas may reject or injure one another.
Track source and lineage
Record seller, date, strain, location code and relatedness when known. Avoid pairing close relatives repeatedly without a deliberate genetic plan.
Prepare separation
Every pair introduction needs a safe way to remove one fish quickly.