Are you performing daily water changes or testing the water? Diluting the ammonia levels with water changes will help. The fins are either a) being torn by something in the tank--unlikely to the the frogs, potentially on objects in the tank like plastic plants; b) the fins are deteriorating as a result of high levels of ammonia. Fin erosion will look much like physical damage in the initial stages. Since this is a new tank, and you haven't mentioned any objects that would catch his fins, b seems most likely. The frogs, just like the fish, are negatively impacted by the ammonia and nitrite levels of a cycling tank, so daily water changes will help all the animals.