You can treat fin rot with metronitrazole. It is found in many fish biological medical treatments, or it can be found in some foods. If you google metronitrazole and aquarium fish, you should be able to find some foods that contain it. Again, average water test kits do not test for total dissolved solids (TDS) which is a measure solids suspended in the water column. Think of walking in short pants and a shirt while someone blows sand at you at 80mph. The air would be the water, and he sand would be TDS. Over time this will cause injury and disease. The solution would be air (water) changes and a reduction in sand (TDS). Fin rot may be a resukt of high TDS. Water changes never hurt. Good Luck...Dwayne