First off, do you have the right requirements for a clam? ( Sorry, you may or may not, I have no idea!)...
Lighting, water quality, some form of calcium supplementation to the tank, blah blah...
When looking for a healthy clam, make sure it's mantle ( the colorful part ) is extended completely, not retracted in any areas.. Make sure there are no ripples or what would look like bite marks on the edges of the mantle.. When a clam is gaping, the intake siphon ( large hole ) is open completely you can tell.. Maxima's usually have it almost completely closed or opened just a tad bit.. If the hole is very large and open, if it looks like it's yawning to you, then it's more than likely gaping..
Also, look for good coloration of the mantle, good reflexes ( meaning that if you swing your hand over the tank or touch it, move your hand by it the mantle retracts...look under the mantle, top of the shell and check to see if there are any white cone looking snails.. ( parasitic snails you explained )..
You'll be fine... I'd buy from clamsdirect.com if you could.. Barry has the nicest healthiest clams I've ever seen.
