Nobody has mentioned cost. Lighting costs tend to be proportional to the number of lamps run and tank configuration may influence that.

In the meantime while your choosing your new lighting scheme, how is your clam doing? Any photos without a flash?

Just for the record, here is my crocea (I plan on adding more as I find better "ultra grade") This is under T5's at the bottom of a 24" deep tank, the lights are 6" off the water surface.

http://i179.photobucket.com/albums/w...lamUnderT5.jpg

Do you still "need" MH to keep clams?

Of course you do. It makes them purdier to look at :)

In all seriousness, no, you don't "need" MH for anything, you could keep them with normal output tubes if you had enough of them. It's a matter of personal taste in what you want the tank to look like. I "need" the crisp light with the ripples everywhere, it's what makes me happy. Key point there is "me". T5 bulbs make other people happy, and if it can keep the clams happy, then go for it. But compare both types of lighting before you make a choice that you might be not happy with.