sudden xenia decline

[Brighteyes_13]

back in febuary(ish) i bought a small frag of xenia (maybe 2.5 inches by 2.5 inches) and watched it explode into a 6x6 sized colony, it was healthy and bushy and extended its arms to sway in the current. over the last week ive watched more than half the colony just disolve! parts of it shrivel up and turn white, parts seem to disolve at the base of the bunch into yellowish goo, and parts just dont extend anymore and shrivel up into a greyish lump. i tried to pull off the worst looking areas, but today, again, more of it has become affected.

any suggestions on how to save the rest of the colony? i tried moving it into a different area in case it was being affected by something in that spot, i tried getting rid of the damaged areas, i tried an extra waterchange.... help

heres some pics of the before/after
Before

after





params
ammonia-0
nitrite-0
nitrate less than 5
SG 1.024
pH 8
temp 78
weekly waterchanges with RO water

the only thing that has changed in the tank in the last month is the loss of a heater about 2 weeks ago. all the other inhabitants seem healthy and happy