My tank's very new. At 2 months, I got a Tiger Deresa Clam. I've had epic battles with my clowns wanting to sleep in it.
http://www.youtube.com/user/HappyHoney41
I'm getting an Electric Crocea Clam, which should show up next Tuesday. I've got to find a good place to put it, because it says they bury in the substrate and I really don't want it doing that. They are both photosynthetic, but I feed the tank 2-3 times per week with phyto. I've heard the larger the clam is when you buy it, the better luck you'll have with it. If you get small ones, they struggle.
I have three

so some tips from experiance:
Make sure you research where they like living i.e.
1. On substrate or on live rock
2. Some like alot of light others are less worried about light.
General tips:
1. Make sure you have good PH 8.0 - 8.5
2. Make sure your temp is stable between 25 degrees Celsius and 27.5 max (can creep to 28 - I had this I still need to get my chiller going.
3. Make sure your other tank mates are NOT nippers (wont eat the mantle)
4. Put them somewhere that they wont get bothered by fish all over them (i.e. clowns

)
5. They don't like to be moved and sometimes move themselves (very little but they do)
6. If you putting them on LR remember they can bore (if they are the LR type) to secure themselves.
7. They don't like alot of flow.
How do you know if they health:
1. If they open once you put them in your tank
2. The light test - put your hand over the light that is directly feeding them if they respond (close or move) then they generally healthy.
3. Look for gaping mouths (you need to find a pic on the net of what gaping is). Now different species"gape" more than others so know your species and make sure you get a gaping picture to compare
Other things to look for when it arrives:
1. Make sure the mantle is not torn or "munched" anywhere.
2. Inspect the shell for any worms, snails or holes (the snails actually bore into the shell and eat the clam).
3. TIP: when you add him / her to the tank hold him / her up right and GENTLY move from side to side, this will release any air bubbles trapped inside of the clam. Air bubbles can KILL the clam.
Other than that you good to go a EXCELLENT addition to the tank.