Can't seem to keep coral alive

I've had my 90 gallon tank for about 8 months it's lit by radion gen 2 I have a 20 gallon sump with a shops protein skimmer rated for a 120 gallon tank an mp 10 and another power head forgot the name and for stocking I have magnificent foxface a yellow tang a flame angel 2 clownfish and a yellow sleeper head goby and for some reason all my coral is not look good my candy canes have bjd I think my montipora are turning white my birdsnest rtnd and my leather is shedding I know that's a good thing but I think it's causing stress to other corals my parameters are

Ammonia 0ppm

Ph 8.0

Nitrites 0ppm

Nitrates 5.0ppm

Phos 0.36

Calc 400ppm

mag 1200ppm

Alk 3.42 meq/l