what casues dead spot at base of coral?

[rsantangelo3@co]

I have a large colony of tri color type acro. it is pretty densily branched and a spot right in the radius of the colony is completly white. Was a perfect square white spot but now its receeding a bit into the darker area of the coral. Was under t5 for awhile now under a single 250 watt halide. Is this caused from light not getting to this area? Like I said not on the encrusted base part but more so in the imbedded area of the dense braches. Prob the darkest area on the colony. No new corals have been added in a long time. all levels in check and running 70x for flow.??? Any ideas guys? Its a 30 cube and no corals in the area. all sps tank with some zoanthids on the floor

 

[davelinks]

sounds like AEFW, if not that, sometimes if i slack on my magnesium, the beases of my acros will die off a lil bit til i get it corrected

 

[fcmatt]

do you happen to have any crabs or other critters that could reach right in an take a chunk
out of it?