Sorry your frog is not doing well . Have you checked pH, Ammonia, Nitrites, and Nitrates lately? What numbers you got?

Hard to tell from description what is the issue. Could be bacterial, protozoan parasites, or internal organ issues after 3 years of being kept in subpar conditions. Myself don't like to medicate unless sure of pathogen. If you want to try a natural mild bactericide; recommend get a bottle of Melafix and treat tank according to label for a week and let us know what if any changes after first couple treatments.

Maracyn 2 is a stronger antibiotic but it works only against gram- bacteria. Myself prefer Maracyn Plus because it works against both gram + & - bacteria and a full treatment is lower in cost. Good luck !