mysterious happenings...

[jennytheleopardgrouper]

Okay, so I've lost 3 turbo snails in the last week. Tested the water, Ammonia, Nitrite and Nitrate all at zero. Brought a water sample to get tested for everything they could think of-everything's great. Something was a little on the high side of normal...can't remember for the life of me what it was...alk maybe? Well yesterday, I was gazing at my tank when I realised that my 2 inch dark gray unidentified sponge had well.....exploded! It was ripped to pieces like from internal combustion....it's now a light gray/white and blowing in the current in strands. My fish seemed a tad itchy over the last week but some garlic seems to have fixed that. Anyone know what could be wrong? I have had some temp fluctuation in the last week while breaking in my new heater...ranging from 75-79 degrees over a few days. Tank is now at 79. I have a light dusting of cyano on some of my CC. Should I remove the dead coral? It looks scary..I'm afraid for the other corals on that rock.

Tank is 120 gallons, 100 lbs LR, lights are 2-48" reg flourescents, plus 2-48" T5's (one 10,000k and one Actinic)
Inhabitants-1 coral beauty, 2 chromis, 1 mandarine goby, 1 new juv clarkii (got him 2 weeks ago), 1 baby regal tang, 1 cleaner shrimp, several crabs (including 2 ugly hitchikers), and snails.
Coral-Show size Toadstool, 3 leather coral, multitudes of star polyps and mushrooms, and one baby bleached anemone.

Any thoughts would be appreciated. Thanks!