Not sure what is happening

Hello everyone. My tank (about 3-4 months ago) had a malfunction. The heater basically over worked itself and raised my temperature to 96 degrees. I added ice and cooler water to help lower it. None of my fish had died, but all of my corals eventually withered away.

Here lately, I've tried to invest in different soft corals to see if they would make it in my tank, and none so far have been able to. I have the API Saltwater Master Kit, and all of my perimeters say that nothing is wrong. I have taken water samples to two different LFS and one of them said that my Alkalinity was high. Other than that, nothing is wrong with my tank. I have a 40 gallon breeder, a fluvial hang on back filter rated for a 55 gallon tank, and also a canister filter rated for a 75 gallon tank. I just recently added another heater to my tank, my temperature is at 78 degrees, which before I added it, it was at 73 degrees.

I have an elephant ear mushroom that is doing fairly well, but my leather coral is shrinking, and my zoas aren't opening. I'm not sure what is wrong with my water. Ive been doing water changes every other week, about 25 percent.

Any ideas as to what it could be?