in some specie, there is no problem at all. Most fish are well and truely inbred anyway.
As well as getting the worst by inbreeding, by culling the bad ones and only breeding off the good ones, you can get a strain of very very similar fish with no (or very few) defomitys.
There will be no stand out examples in your tanks, just sameness.
To get the best in raceing pigeons, we inbreed two families and then out cross to one another. The offspring are normally great racers but terrible breeders