Without knowing water conditions or seeing frog this can become a guessing game. Still, based in my experience, one adult ACF in a 10 gallon tank should have a well established filter (even if just an internal air driven sponge filter) to reduce the toxic effects of ammonia and nitrites. Weekly water changes are probably not enough and your frog might have been suffering the effects of these toxins for a long time. Chronic exposure to them can cause kidney and other organ damage.
Recommend you change 50% daily with same temperature dechlorinated water and get a filter for tank. Once filter is running, change 25% water daily until filter is seeded with bacteria that will reduce the ammonia and nitrites. Seeding cycle will take 30 days; but can be cut in half by using a product like Seachem Stability. Once filter is cycled, reduce water changes to 25-50% weekly and clean the filter media in water from aquarium during water change. Hope the increased water changes help your frog, good luck!