Hard COral Diseases?

[Nexus13]

tank is over a year and half old

the hard corals that died have been in there for over 5 months

all my parameters are good (or so i thought)
pH = 7.9
Ammonia = 0
nitrate = 0
nitrite = 0.1
phosphate = 0
calcium = 380mg/L

yah all my fish are good,
soft corals are fine
just the hard corals all died very quickly.

i looked it up on the net and it sounds like
Rapid Necrosis.

anyone else had or know of this?

in terms of changes:
i did add a neon dottyback
and some porcelain crabs that didn't make it.