Clams that won't attach

[Delphinus]

I already posted this question (well a similar question anyways) in the clams forum at RC, but, I want your guys' opinions too please.

I have trouble getting clams to attach to anything. Pretty much any clam I've ever tried ... probably getting close to about 10-12 over the years ... one common theme amongst them are that they never ended up attaching to rock.

Now, one clam or two, I could beleive I bought a clam not predisposed to attaching (perhaps byssal gland injury). But all of them? Come on, can anyone be that unlucky? More likely that I am just doing something wrong.

If you have a clam that wasn't attached to anything when you purchased it, but subsequently attached to hard substrate in your tank, can you please share with me your guesses as to why it worked?

I'm interested in in things like placement, was the rock level? was it at an angle? was it it strong current - medium or low current? Did you add sand around the base? What kind of texture did the rock face have?

I'm sick and tired of $5 snails knocking over $50 clams at night. This is rediculous. At least once a week this happens.

The clams I have right now are T maxima. Tank is a 75g (48x18x20), lighting is 2x250Iwasaki+2x110 actinic, Ca=>400ppm Alk=4.0meq/L, NO3=0.0, PO4=0.0, I dose DT's & ESV's daily at roughly twice the recommended dosages.

Sorry for the long-winded question. Advice appreciated. Thanks.