with Anual Killis (Nothobranchius) There enviroment drys up every year so they are all closely related.
In Korea and experiment was done useing raceing pigeons and breeding mother to son, then the female from that mating to its father and so on. After about 12 (or 24 I cant remember) generations, there was a marked increase in problems with any deformations (Physical and mental) showing for the next few generations but by selective breeding, these were culled out and sameness became the norm. No racing stars or good looking birds just sameness. In fish (being non preforming) it may not be a problem and in fact may be an advantage but where preformance is needed, its to be done with careful consideration. By crossing 2 line bred families, you can often get increased preformance through hybred vigour but you would never breed from them.