The discus came to me parasite-ridden and soon had damage to the dorsal and ventral fins. Once the parasites/possible bacteria were eliminated the fins healed but only up to a point. I cut back the damaged bits when it was clear that the fins were not going to grow in further. The re-growth was rapid. While it is possible that there is another explanation for the fin damage, it wasn't genetics, nutrient deficiency or internal parasite. I have no reason to think my water was an issue.