I recently became the caretaker of a 225 gal tank. I have been taking care of the fish for 4 months. It has 3 10'' oscars, 1 adult firemouth cichlid, and 3 plecos. The tank is overstocked and underfilterated and in poor condition and I am not experienced with fish. I just felt really bad for them and thought I would try to save the them. We have been doing weekly waterchanges but the nitrates are still really high. We are planning to get another filter and get rid of 2 of the three plecos but we are trying to keep them alive until then. The largest pleco is about 11" and 3 weeks ago he developed a white slime behind his eyes on his head. None of the other fish had the slime. We took him out of the tank and treated him with TC Tetractcline for two weeks. Yesterday we put him back in his tank and he was very agitated. We then noticed that he still had scaring from were the slime was( second picture) and he also has a large hole in his dorsal fin (first picture). I do not know if it is an ulcer or he got into a fight. Help.