dying ?

How long has this particular coral
been in your system?- could be too intense lighting like someone else mentioned. Also, I'd worry more about KH than the Ca, 500 and a bit over isn't going to kill anything but even LPS don't like KH that low. Chances are if you get KH up to where it should be(slowly) the Ca will drop a bit.

Get yourself a Quality Mg test kit. Mg will have an effect on both KH and Ca.

 

are you dosing alk and cal or run a reactor?your alc is way to low

 

I'm of a mind it's something else besides your alk/ca. My alk ran at about 6.4 for weeks with no ill effects on my corals. At the same time my ca was 420 and mag was 1320. I know it differs from tank to tank but still.
You stated your new water calcium is testing at 500 right? Thats unusually high. What are the manufactures readings?

You may want to cut your light time down to 8-10hrs daily. Especially the MH to no more then 8hrs.

 

I have had rc and oceanic 500 cal right out of the box oceanic specs are 500 cal but alc is lower than rc

 

I'd definitely take a look at getting a Mg test kit and looking to keep all 3(ca,alk,mg) consistant.

 

I am trying to find a Mag test, I live in fort mcmurray, if it isn't in the shop, then it doesn't exist, "except maybe in the city" LoL. I have moved rocks and corals obout and cut the light halide down to 5 hours a day. My mushrooms appreciate this, that's for sure.