what are the symptoms of a old fish

i recently had a oscar that i have had in my tank for about a year, start acting odd, it would go berzerk, swimming out of control, into rocks, the top, other fish ect, leaving gashes in himself, and it wasn't eating, i hand fed him/her night crawlers, and determined it was blind, but that was not always the problem, i got this tiger oscar from a guy off of craigslist that was moving, and he gave me like 10 huge oscars, this guy was really active for about 8 months, then the odd behavior started, and progressed, well on Friday he/she was having a really bad time swimming/balance/ was bloated on one side, i am hoping he/she was just old, i made the decision to kill him/her rather than see it suffer, for a fish lover, it was really difficult to do it too, i guess what i am asking, is...has anyone else had, or is having similar behaviors with their oscars ?

That sound more like an illness to me maybe a swim bladder infection? As far as signs of them getting old not sure about oscars but most fish kinda just slow down and don't eat as much

Oscars live a long while. Do you know how old they were when you received them?

hmmm with behavior and symptoms like that i would strongly associate the cause with poor water quality, not old age. when the water quality is bad the fish go nuts literally trying to get out. size of tank and number of fish have serious effects on the quality of life on such a large breed of fish. oscars are big and messy fish.

what do the results of your water tests say? whats the size of your tank with as many large fish you have?

btw how is that 500g tank of yours? your oscars shouldnt be behaving like that in a tank that big. not enough filtration maybe?. post pics