In my view,, as well as others, inbreeding as well as cross breeding,, is primary reason fish strains are becoming weaker. Inbreeding produces same weak genetic traits that parents may or may not posess and these traits continue to be passed on to generations of young.
Always best in my view to select the best of fry and remove them to breed with fish that posess the desired traits whether it be color,finnage,etc. and not allow fry to breed with fry from same generation.
Crossbreeding with cichlids for example , results in fishes with unknown genetics, unknown health issues, and unpredictable behaivor along with possible shortened life span.
These fish ,like livebearer's are aslo capable of passing on weak traits as well as genetic developmental issues and is why in my view,, more and more fish do poorly in the long run compared to fishes of even ten years ago.
Breed strains of fish that are true,and don't inbreed. This way ,others will be able to purchase healthier fishes.