Daily feeding is one big part of the issue...along with having some really big fish in there. I feed three times a week and half a cube of whatever frozen I'm alternating with. You want all food to be eaten in a minute or two, and then remove any still remaining.
What kind of triggers? I'm not aware of any that can live happily in this size of tank, since a standard 75 is only a wider 55.
Having fish oversized for a tank leads to overstocking. Tie that in with your rock not being porous, so there isn't much area for beneficial bacteria to grow...isn't helping matters. Big fish = big poop = big bioload.
For water changes, 10% weekly is good maintenance. But what does it really mean in terms of helping you understanding your issue at hand? If your nitrates are at 80, a 10% water change will only take out 8, leaving you with 72...which will then increase until your next water change! A 50% water change would bring you down to 40.
To get that number down to an OK level for a FOWLR
system, I would do large water changes until the nitrates are under 40. I would also remove the triggers back to your LFS
. What other fish are in there? Check out liveaquaria.com. These guys have really realistic tank size recommendations for fish.