My issues with coral over the years have stemmed from the following:
too much alk, too little alk, too much alk swing, too little alk, so yea, 80%+ alk issues from me not testing for a month or doing something stupid like leaving a doser on or putting alk in the calc doser.
other than that, i had a long stint with water in my system going septic from stagnant flow in pipes/chiller etc. That one was a NIGHTMARE to finally figure out. Things just die randomly. one day i smelled sulphur and after that it was good.
another time i had a long stint with TOO LITTLE nutrients and things would just slowly pale and die over 3+ months. had that problem for a long time too. go over on nutrients vs under is my current paradigm.
lastly i've had a couple times where calcium was 350 ppm which corals didn't like (again usually due to not testing for too long)
after that, throw in lots of rounds with pests, mostly AEFW being the problem. Red bugs are annoying but really takes a long time to hurt corals.
once i even had a small tupperware container in the tank that was about 5"x5"x2" that nearly nuked all my acros. went septic inside it from no flow but ammonia test still read dead on zero. I blamed it on several other things before I finally discovered the tupperware flow got pinched. removed it and things were better 2 weeks later.
I'd say get things back to dead on and keep them there for a couple weeks. If you don't see improvement then go looking for stuff similar to above. @Danh
and @MyAquariumPro I don't have the 3rd pump in hand yet. it's being shipped. I'm currently in design for a closed loop. My overflow is only a 2" pipe and can't take more than 2 reeflo hammerheads lol. Unless I make the backup pipe full suction also and emergency the overflow, but then I have no emergency line so i'll probably go with closed loop. It's tough hiding the 2" pipe and getting a return into the tank without drilling though.
Picking up a MP60 from JJocean tomorrow morning as I need more random flow. too much is coming to rest on rocks or just beating corals down with constant powerful flow. Gonna look into some sort of additional flow with the new pump, maybe sea swirls or tunze mounted on posts that rotate.