plate and brain coral woes

[spragro1]

my long tentacled plate was doing great until today when i noticed a pie shaped wedge of completely dead tissue - skeleton is showing - also my brain coral is showing its skeleton as well - although the brain is very new - couple of days old - 180 gallon tank - 2 400w MH - water paramaters are fine except for a little nitrate spike due to a deceased clownfish - also my devils hand suddenly turned black and melted - what is going on? Have crabs/snails/ 2 tangs a clown and a couple of damsels plus i sea urchin - could the urchin be stinging? I am very frustrated with corals