pineapple brain discoloration - dying?

[klwheat]

I have this pineapple brain that I have had for about 8 months. It has always looked beautiful. Recently, I noticed a discoloration around the bottom edge of the rock formation. In the last 2 weeks, it seems to have spread. (pics linked below - one in full light, one in actinic only).
I have no idea what is going on with it. My water parameters are same as always - pH 8.1-8.2 depending on time of day, no ammonia/nitrates/nitrites, calcium 460, SG 1.025, phosphates 0.05, alkalinity 4meq/L, temp 79-80. Then, I did 20% water change (100 gallon aquarium with 29gallon sump). 1 week since and no change.
Any ideas?? Everything else in the system is absolutely flourishing, so I'm kinda at a loss here.
TIA for the help!

https://picasaweb.google.com/lh/photo/CsvXY_7luCp0paG3bJvtj5RvFLnJbl3rYDoj5O7qVoQ?feat=directlink
https://picasaweb.google.com/lh/photo/FhK43xz-WkTRpCSdkSdFfZRvFLnJbl3rYDoj5O7qVoQ?feat=directlink

 

[NC2WA]

since water parameters havent changed, then do you have any fish that might be picking at it? Do you have high flow hitting it?

 

[klwheat]

The only fish Chang has been a new blue hippo tang, and I've never seen any fish near eating it. It isn't in really high flow, but the same place its been for months. The only other change was to a finer sand a couple months ago. Someone told me that the finer sand can irritate it if its sitting on it, so its up on another rock now.

 

[Faciosity]

What I'm seeing is not discoloration, it is dead skeleton. Corals often recede from poor water quality, injury, lack of nutrition and/or light, or from chemical or biological warfare. If you cannot get the receding to stop, your best option to save part of it is to physically remove the dying parts (this can be done easily with a wet tile saw or band saw).
I have owned lots of LPS corals, and from my experience I notice that they have a really hard time growing back over the jagged skeletons that they leave behind, so I would remove it anyways to promote new growth.

What kind of light is it getting? Have you changed your bulbs? Could it have been stung in the recent past by another coral? Any possible pests?

 

[klwheat]

I have 4 x 48" t5s 10k and 2x 48" t12 actinics over the aquarium....no changes in that department in months. Bulbs are about 5 months old. I don't really have any other corals near it, and haven't moved it. What kind of pests might I look for?
Thanks.

 

[returnofsid]

It's definitely flesh loss, receding from the skeleton. My guess would be the change in sand. Are portions of this coral getting covered with sand? I've had similar corals lose flesh, from that very reason. Also, the parameters you mention look great, but you didn't mention Magnesium. Magnesium is important, to this type of coral, for flesh growth. If Magnesium is low, it could be causing this.

Get your Magnesium tested, and your target number should be 3X your Calcium.
Make sure it's in an area of decent flow, but not being completely blasted.
Make sure it's not getting buried in the sand, at all.
If you see areas that are appearing slimy, make sure to blow that slime off, as it'll cause the recession to spread faster.

 

[klwheat]

I will have to find someone to test the mag...or go get a test for it. I've been using Kalk +2 (with mag and strontium) for my ATO, and i've never had to supplement calcium...never checked mag. The placement is getting good flow as you mentioned...but not blasted, same place its been for months. Yes, it was definitely getting sand on the bottom edges, so that's now elevated and cleaned off. Guess I'll keep an eye and see if that helps.

 

[returnofsid]

If the bottom edges were being covered with sand, I can almost guarantee that was the issue. Just watch it now, to see if the recession continues.