I have not done this myself, but I have some ideas, if you like.
First off, it should be set up just like any other reef, with a live sandbed and lots of live rock to maintain water quality. If you're worried about sand critters, you can forgo the live sand and use dead sand. Live rock shelves are nice for maximas and croceas, too.
Also, there's another thread here in the clam forum about tanks specifically designed for clam viewing. Check out that thread for details.
Also, you might want to consider some fish for nitrate production for the clams. A small school of harmless fish such as chromises might do the trick.
I have heard of people with clam-only tanks supplementing the tank with ammonium something-or-other, but I cannot recall it. If you want to set up a clam only tank, I strongly recommend buying Knop's book on giant clams. he covers all these things in that book.
Also, you will want to consider hammering kalk, as a tank full of clams will deplete calcium very quickly. Trace elements will have to replenished as well. A good trick is to blow a fan across the sump to increase evaporation so as to be ble to introduce more topoff water (and thus more kalk). Of course, this will lower the temperature, so you'll need to adjust the heater accordingly.
Just some ideas...
