A few years ago I posted a link to a study done that demonstrated corals are more resiliant when raised in an environment that had varying temperature.
The takeaway from the study was that corals need to be raised in varying conditions from the start if they are to survive varying conditions in the future.
Established corals raised in tight parameters did not survive when subjected to the new varying conditions.

One of the consequenses of aquarists trying to maintain long term success with corals by maintaining rock steady lighting, water chemistry and temperature is that when corals raised in a steady environment are subject to changes in those parameters, the corals are not strong enough to survive those changes.

In nature, corals are subject to environmental changes including lighting, salinity and temperature.
Our commitment to keeping corals within tight parameters produces fragile corals.

The best you can do for your corals is learn what conditions they were kept in previously and don't let your tank conditions wander too far from those conditions.
It's neither easy nor simple to do.

Here's a link to the study:
http://onlinelibrary.wiley.com/doi/1...199.x/abstract

The title of the study is "Effect of fluctuating thermal regime on adult and larval reef corals" Putnam 2010 Invertebrate Biology


And a free article from Dr. Shimek on the subject:
http://www.ronshimek.com/salinity_temperature.html

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I've definitely noticed this over the years. My old 90-gallon reef with halides over it used to fluctuate from 79-80F at night to 84-85F during the day. It did great. Would it have done better if I tightened up that temperature swing and kept the peak lower? Probably. However, the corals were very resilient. :D

I think this also true of other parameter swings too, such as KH.

It's frustrating when you check all parameters which check out fine, but your corals are still dying.
I think reviewing the past conditions that the corals have been exposed to is more important than what the current conditions are.

Sometimes of course, changes are too great for even strong corals to withstand.
Think of what's currently happening to the Great Barrier Reef.

Good info. Thank you

Thing is, I haven't really changed the parameters too much. They went up slightly but that was over the course of weeks so it shouldn't have bothers them right? Only real drastic change was the t5 to led but that shouldn't kill every coral in the tank.

I'm still thinking it's the sand bed that's releasing hydrogen sulfide because of the smell. I decided to remove half of my sand bed last night and the water and smell was disgusting. I'll post a pic later for you guys. I'm mixing more water right now to remove the rest of it.

Not the best move to remove your sand bed when the tank is recovering from a crash, next time have your water ready so you can do a large wc at the same time, people usually skim the top layer when removing their sand bed 1/2" at a time...

I don't have much else to lose l'm down to a few spa frags now lol. I syphoned out my sand while I did a 50 gallon water change. Matched all parameters in the tank before I did the change.

I checked it out this morning and everything looked ok. First morning I didn't find anything dead. Fingers crossed!

Your corals look to be a large size, so maybe think of what conditions they were kept before this new tank setup.
H2S can be removed from the water with activated carbon or by oxidizing with ozone. Hach makes a kit for measuring H2S, but it's best of course to reduce sediment buildup in the first place.
H2S is deadly to aquatic animals as much as carbon monoxide is deadly to humans. It doesn't take much to be fatal.
Good luck!

When you do your WC, do you typically siphon the sand bed on a regular basis? Maybe I missed it but how often and how much of a WC do you do?