Breeding siblings for one or two generations isn't really a big deal. The inbreeding is easily corrected with one generation of outbreeding to an unrelated fish. The danger occurs when you breed several generations of siblings, so that you have fish getting two copies of defective genes and ending up with mutations that can cause physical deformities and health problems. The same concept applies to every species that produces sexually, including humans, dogs, cows, etc. Genetic variation corrects this, because with many of these mutations, if a healthy copy of the gene exists, it will override the defective copy.