Actually, I believe the tails are being nipped by other guppies. Mine do it too. Fin rot is normally bacterial, not fungal. You can treat both tail rot and ich by raising your temp to 86F and add one tablespoon of aquarium salt per 5 gallons of water. After initial dose, do daily water changes of 75% and add 3/4 tablespoon of aquarium salt for every 5 gallons. Be sure salt is dissolved before adding to tank or it will burn the fish. I know big water changes are a hassle in large tanks. I just did the same think in my 55G recently for a bacterial infection. This is much better to do though and no meds to stress fish. Also vacuum the substrate every couple of dats to get ich out.