Embryo viability was significantly reduced in inbred exposed fish and there was a tendency for inbred males to sire fewer offspring. ... The effect of inbreeding on reproductive behavior was studied in the poeciliid fish Heterandria formosa.

Can you inbreed fish?

Yes, a fish can interbreed as long as it lives in the same community. It isn't a matter of concern for a fish to breed within its families. Unlike humans, it won't grow up with any deformities.

Is it bad if fish inbreed?

Even when fish are raised for food, the inbreeding produced by small populations can cause problems. If unwanted inbreeding occurs, it can reach levels that cause growth rate or other production phenotypes to decrease.