[Coral-List] Question with Acropora tissue loss

[Damien Beri]

Hello,

Having some trouble identifying what's causing tissue loss on some Acropora and montipora in a large 700 gallon aquarium.

I am attaching images of corals, an ICP-OES test.  I am also attaching an image of what looks like some type of snail on the recently dead part of the coral.  I have no idea what is going on in the tank as most of the chemical parameters seem fine.  Curious is the coral images show signs of pest presence.

https://drive.google.com/open?id=0BxzVF083TtxMLTA2dm9udGFWNzg

We use 4, 400 watt 20K metal halides, and 4 aquatic illumination 52s set to max blue setting.
Tank sits at 74-76 degrees.  The calcium is up at 450, alk 8.3, and Mg 1350 as of today.  I am still seeing tissue loss. Phosphates and nitrates are very low as well.  The ICP-OES test did show high phosphates but that has been corrected. 

We have been unable to isolate the exact cause of Acropora loss, and are even stumped because some corals (Acropora and montipora) are completely fine and thriving. All of the LPS corals seem fine as well. 

Necrosis progresses from the base of the corals, and maybe seems to occur after a water change of about 20/30%.

Thanks,
Damien Beri






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