hi Dragonette
I'm sorry about your loss. As others pointed out, though, your tank does look beautiful and it's clear that you take good care of it.
I have a Derasa, 4 Maximas, and a squamosa. In my opinion, your problem w' the Derasa stemmed from one of these 3 reasons...
1) Maybe too much light/heat. It's a small tank, and the heat & light from your MH might have stressed out that deeper-water clam.
2) Chemical warfare: With a tiny tank, there's lots of chemicals getting emitted from your other coral. Perhaps something (like that torch coral) disagreed w' your clam, stung it, and rendered it lifeless. This seems a bit less likey- given the relative peaceful nature of most of your coral...
3) Night-time predators. Like someone else pointed out, you might have some worms in your sandbed, pyramid snails, or some other predatory nocturnal invertebrate. Clams have sturdy exteriors, but are vulnerable at their foot. If any hungry critter is residing in your sandbed, it might have attacked the Derasa.
I hope this helps. If I were you, I'd just wait on a clam, and focus on something else cool (like an Aussie Acan, donut coral, or another type of LPS). By introducing another one you're risking not only that clam, but all the other coral in your tank. As you quickly learned, a decaying clam creates a foul stench and can pollute your tank (esp. a little tank like yours) in such a harmful way.
IMO, the upside doesn't outweigh the high risk.
Best of luck w' your tank, and, again, sorry for your loss.
-Flamehawk
but I am in love with torches so its hard to give them up. 
