Very important question.........these are two very different ball games. Another very important question is how long these SPS have been established? If they are fairly new....well that changes my answer as well because some SPS just do not do well in a captive reef.

I agree that old bulbs will likely not cause RTN....but loss of colour. I dont think calcium would be the culprit though either......alk maybe....either out of wack or not stable. Other possibilities is a nutrient problem or some kind of contaminant in the water.

white as in flesh disappearing and skeleton showing. They all have turn super bright white on the first day, then starting going yellow brown by the 3rd - 4th day. When they are brown, there are about 20 hermits that show up and chill out on them.

The first large one to go was from a tank tear down of reefer rob i believe. I got two corals from him, the second one is still going strong.

The second large coral to go was from oceanic corals thats was in my system for several months.

The last two are from whatcaneyedo, they are smaller but were really doing well.

ill have to check the mg, as I havent checked it in awhile.

I'd blame the bulbs. I had the spectrum on both my MH 150's go at the same time (thus I didn't notice it because each looked identical) until 2 Acro's bleached whiter then white. After a week or so they turn into a algea farm :)

Death of a single coral/colony will sometimes spread to nearby corals. This has led me to conclude that there may be a bacterial component at work.

I'll provide an example...

I was running two isolated systems. Both had healthy populations of SPS. A colony began to die in tank A. So, thinking that tank A had some kind of problem (lighting, CA, Alk etc.)... I moved the colony into tank B.

When I returned to tank B less that 24 hours later, 4 SPS closest to the imported colony had died.

There was nothing wrong with any SPS in tank B. The addition of a single piece of dying coral killed others within 24 hours and it killed corals closest, first.

It has nothing to do with tank parameters.

WTH!
I just lost another SPS. This one was 12 inches directly under a 150w MH. White in under 24hrs. It was about 6inches round.

Im thinking I better get the new light over this tank ASAP.

had that happened to me few months back.

I believe it was the low alk cause the initial die off. And since I was away from home during that time, more SPS was dying. Came from from holiday, only to find that I already lost 1/2 of my SPS and die off continued for another week of so.

not sure what fix my it. since I did a multiple thing at that time

1. lots of water changes
2. correct the low alk issue
3. add in carbon reactor

hope any of these helps

Going with this as well. It's not the light, which should be changed anyway, and Ca and alk are not at levels that I would consider harmful. I've run my Ca close to 500ppm before, and alk anywhere between 5 and 14dkh is fine (as long as it's stable). I've had colonies that just go, and often take others with them, even when moved to another tank. Anytime this happens now, I quickly remove the colony, try and salvage a frag from the tip furthest from any recession, and pray to the reef gods. Sometimes SPS just do this, and too much mucking around with tank parameters can make it worse. Go for stable water, get a new bulb because you should have last year, and ride it out. A nice water change never hurts either.

Well I consider my parameters stable but within the next couple months ill have my new fish room up and running so that should be a extra few hundred gallons to stable it out.

What i dont get is all these colonies that have died were flourishing the day before. Just doesn't make sense. The one that just died was a small 1 inch frag and it grew into a 6in ball in under a year. SPS is starting to drive me nuts. None of my LPS/softies have died

Welcome to the club :) IME, when one goes poof, it often takes others with it. In my tank I have about 80 pieces of acropora. One day I get up, check tank, all's good. Go out for the day and come home to a large colony completely gone. Just went poof. Everything else was and still is fine. Who knows why this happens, but again, IME, mucking around trying to fix an unknown is just asking for more trouble. Do a good water change, clean your filters/skimmers, etc, and replace carbon. Then hope it stops. I've gone through this and lost half my tank, then everything was fine.

From my experience, sps rtn isn't caused by pests because no animals can consume a large colony in one day. I had some sps rtn before, some were wild colonies and others was due to heat in the summer with temps rising above 89f. I also had a tank crash because of meds and disturbing the sand bed. But since you don't have wild colonies and run a chiller there's definitely something in the water. Have you been dosing anything new when you started to notice the symptoms of rtn? Something in the water that the sps don't like for them to die off so suddenly. Hope you get the problem rectified soon, I would say like others have suggested maybe some water changes to dissipate whatever is poisoning the sps.