I would start mixing water, and do a water changes every few days until you get your nitrates back under control.
Nitrates alone will not immediately kill your fish, but they are a good indicator of general water quality.
I neglected a 75 for a few years, and it took about 4 30% water changes to get the nitrates back under 20.
I did these over the course of about two weeks, as I let all new water mix for a couple of days before adding it to the tank.
You might also try some frozen foods, just rinse any that look cloudy once defrosted.
And get some seaweed sheets for that tang. And before long, it will be time to get him in a bigger tank! Actually, doing that now will help you with nitrate levels. You should try selling or trading him if you don't have a larger tank for him.