Acan's peeling, white teeth showing...HELP

[jjj1100]

Hello,

Tank has been set up for almost 2 years.
parm's
Ammonia 0
Nitrite 0
Nitrate 20 (has been that for 2 years)
The following 4 from salifert kits
Calcium 400
Alk 13
Mag 1100 ( I have been dosing to raise to 1250)
Stron 15

2 months ago I changed all the bulbs in the tank and cut back on the lighting to let then corals adjust. Slowly after this I got a brown film algae that began coating my sand bed. I keep all of my acan's on the sand bed as well. All of the sudden I began seeing white skelton's where the acan polyps were. A few of the acan's have been in the tank for 1 year. I lost a few frags in addition. I have added 100 blue leg hermits and they took care of the algae that was coating the sand, but I am still seeing some recession on the acan's.

I haven't added any fish that nip or anythng like that and I have been staring at the tank/fish trying to see if anything even swims near the acan's ....nothing.

Is there something that could be doing this, something esle I should be testing for??

Thanks in advance

 

[All Delight]

If the tissue is peeling off and flapping, I've never had them come back. If the tissue is receeding, then dip it in Tropic Marin Pro Coral Cure.

 

[ServantSoldier]

what is your PH and salinity?

 

[jjj1100]

salinity .25
ph 8.2..ph doesn't change at all as I run the 110g fuge opposite hours of the display for lighting,

 

[otiso777]

Do you target feed them at all?

 

[jjj1100]

no target feeding, some of these are 1 year old and just catch food as it flows down in the water column as they are all on the sand bed. Which leads me to think it may be the sand bed, as its 2 years old. I am going to change half of my sand bed and see if that helps

 

[atf72]

If the polyps are peeling back or bailing off the skeleton you're in trouble. I lost numerous colonies and could not find the answer. I ended up thinking it was some sort of skeletal bacterial infection, but never figured it out.