I did change and it seems still nitrate is high. Don't know other ways to bring it down. Big water changes are not good As far as I know , he still in the corner but now in a different one right next to where filter is40ppm is much better but still alot higher than I like. Also keep in mind your fish is stressed and thats all the more reason to keep up with more and more fresh water. if you get down to 5-10ppm nitrates and he is still not doing good we can try something else.
In the mean time I would suggest dropping the temp slowly down to 73-75 and add some airstones to ensure good gas exchange. Use prime to detoxify.
Feed very little too, and keep lights on for 6-7 hours a day and dim if possible. creates a more calm area.
DON'T change out everything but rinse it off in old tank water and reuse it. The only thing that should be swapped out is the ultra fin filter media like floss. or things like carbon if old.
You don't want to remove all the beneficial bacteria and kill your cycle. that would make things far worse.