Sick Snow King

Question
I've had my Plecostomus possible snow king for a couple of years i got him when he was about 3" he is now about 11. he lives in a 50 gallon tank with 2 other gold fish and an albino tiger Oscar. recently he hasn't been eating his food has body sores on his chest area, his is getting very thin and his fins are starting to tear and he isn't sucking on the glass anymore he just sits on his side. he has been acting this way for about a week and i have been trying to research but i cant find anything. can you help me out please i would greatly appreciate any help!!!

Answer
krystina,
check the PH and nitrate levels.
look for ulcers and open wounds he may have injured himself and infection has set in.
you may have to administer antibiotics at some point, you could isolate him in a hospital tank as well at this point.
Let me know the findings.