Corals Receding Help

Iv noticed a bunch of my corals are receding from the base up. I have 3 different milli's receding, a tri-color acro, steve elias stag, pink jade, lokani, and a hawkins echinata all receding from the base up. They started receding at different times over the last couple weeks. I have cut some up to make them stop. Some of the cuttings are still alive. I went camping last weekend. When i came home i noticed the tank didnt get its top off water or kalk. The ph was 7.8 which is .2 less then normal for my setup. I checked calcium and it was low, about 320. So i raised it a little and started kalk a little stronger. Calcium is now at 400.

My water parameters seem to all be in check and im still noticing new corals starting to recede. Just noticed the stag receding today and it has gotten worse on the milis and acro's.

What can i do? What can cause base up receding like this?

 

Get your pH up into the normal 8.0+ zone. Falling below 8.0 is more acidic water, which is probably the big cause of it, assuming no other problems like fungal infection. Get more oxygen in your tank, use an airstone or open up the air in your skimmer. Remove any glass canopies that may be on your tank top (assuming you have that), or let some fresh air into your house.
You should also check your Mg and Alk levels, not just your Ca. All 3 elements need to be balanced for happy corals.

 

Sounds like STN (Slow Tissue Necrosis). If the tissue is receding, you will want to frag from above where the recession is, seal the bottom with super glue, and then remount the living portion