datman
I noticed you have a 50g tall with 250 MH - how far away from the lights was the clam, what type of clam was it, what size was it, were you dosing strotium or iodine, what kind of shape was the clam when you bought it, were you doing any supplemental feeding with Phyto, what were you water parameters, how old are your bulbs, etc.
Start by buying from a good dealer - you can't go wrong with Barrys or East Coast. For your first clam get one that is 5"-6" so you won't have to spot feed - but you will still need to dose DT's about twice a week (turn off your skimmer for about 2-3 hours). Run your temp about 80 and salinity at 1.25. Make sure your calcium is 400+, dkh 10, and ph 8.4. Start by placing the clam about half way up the tank in a low flow protected area - if it doesn't jump off and is doing well move it higher in about a week. If it decides to jump off the rocks 2 or 3 times then try placing a flat rock on the substrate and see if it will stay there. If it moves around let it go where it wants. Be aware - if it's on the bottom of your tank I would increase the DT's feeding to 3 times a week. Do not dose strotium or iodine (poison to clams). If after 6 month's it is still alive and growing then you are ready for 1 or 2 more and you can advise someone else.
IMHO
Joe