The first and most important recommendation is to reduce the frog handling to the absolute necessary when cleaning or transferring frogs. Leopards are shy animals and will be easily stressed, which can lead to disease. Those constant "examinations" can do more harm than good .

If the waterfall filter is running and cycled; you probably need to do a weekly 25% water change. Clean the filter media in the water you remove during water change; not in tap water that will kill the good bacteria. Frog looks healthy, can also try to feed not dyed night crawlers from Walmart or local bait shop. More info on leopards in here: Frog Forum - Leopard Frog Care - Rana pipiens (Schreber, 1782) and R. sphenocephala (1886); good luck !