Hi, after having an issue with cardinal tetras in a new tank and losing 4 out of 6, weāve managed to keep our other two with daily water changes and they were back to good health albeit a bit lonely. Anyway all our ammonia and nitrite levels were good so we added some more cardinals at the weekend and Iāve noticed one doesnāt look too good. It has a sort of lump on the top fin and I have no idea what it is, Iāve attached a photo.