HELP! Coral melting, snails falling off glass.

In the span of just 10 minutes, all of my mushrooms, zooanths and my xenia and my anemones all curled up and melted, and at the same time, all 10 astrea snails fell off glass and rocks! My lawnmower is ok still.

4 month old 12 gallon nano:
0 ammonia
0 nitrite
~0 nitrate

Nothing was added to or around the tank at the time. It occured actually right before I was going to do a water change... did a 50% in response.

No soaps, household poisons in contact, no feeding for two days, and nothing was around the tank before the fallout.

-What to do?
- Any one had this happen?
- Why?
- Is there some toxic coral in my aquarium causing this?

Last thing added was two days ago:
-tiny little bit of silverside for the polyps from the same bag I've been using for months.

No, wait. Last night I topped off the water with the normal RO water from equipment that's never seen soap or anything. I rely on frequent water changes to keep up the minerals, so there aren't any additives. No new corals, no new fish, I even checked the UV blocking plate on my lights and that's ok, too.

I added a small bag of activated carbon in a filter cartridge to hopefully absorb whatever it is that's causing this.

I appreciate your help!

Thank goodness, no problems with the equiptment - checked powerhead for oil leaks too. The corals are doing better, some of the snails are still falling off, and the xenia is still tightly closed. If it gets worse again, I'll assume it's a reoccurring thing in the water and do another water change.

Does anyone know if corals will ever "bomb" tanks like this with toxins they release?