Well id say its a combination of factors, certainly those high nitrates, plus not enough light, and/or wrong spectrum, and the high temp likely not helping either. Need to get those nitrates down to 10 or less. Ive seen some tanks do well with nitrates in the 20s but generally after that it starts having a pretty dramatic affect on the health of the inhabitants, especially sensitive inverts.
How old is the tank, and what is you current filtration setup? Whats your feeding schedule like currently? What bulbs do you have in those t-8s?
Really a few ways to deal with it, and a combined approach would be best, but first, and prolly most important, would to identify the source of the high nitrates and get it under control. Second is to increase filtration. After that I would ditch the t-8s, and either get t-5 vho, or another led fixture.