I have a 10g w/ a derasa in the bottom and the pc lighting fixture you're talking about. I have had it for atleast a month now... and going strong.
I set the tank up maybe 5 months ago so it is a new setup, HOWEVER, I got the rock, sand, even the water and some corals from my Display tank so it was established from the get go.
I do agree w/ mbbuna, if it's not established, hold off on clams...trust us
yes, I do have to top off ALL THE TIME