clam placement

Ok I wound up getting a clam at the frag swap. I conned Dale out of it after he bought it from Paul. I'd been eyeing it all day and was about to pull the trigger when I saw it was missing. But I found it.

Now my problem. When I got it, it was attached to a shell. I placed it in my rocks and it opened up imeaditley. So I'm thinking all is good. Well the next day, it's not opening as much and I noticed that every time the powerhead that was near it turned on, it would close up. I also noticed that it had let go from the shell. I was going to move it but the next day, it was MIA.
So yesterday I took my rocks apart and found it. I moved it farther away from the pump and in a more secure area. The clam was opening once again but not fully like it was in Pauls frag tank. This morning, it had once again moved off the shell. I'm not sure if it's moving or something nocked it off. But I haven't seen any of my fish bother it.

So my question is, should it go in the rocks or on the sand? How much flow? Does it need to be in a constant low flow like down on the sand? Thanks for any help. I'm hopeing I can finally get a clam to live in my tank.

I also got one from frag show and it also came from the shell it was attached to, he has now reattached himself to a live rock and location is mid of tank at highest point of live rock. My tank is 48"x17dx16h about 4 from water level. This morning he is fully opened and looks good.

My clam is sitting on the sand bed but my tank is only 24" tall.

My clam on the sand

Yea, he "jumped" off the rock again and I found him in the sand. But he was open and happy. So I moved his "shell" down to the sand in front so I could see him. So far he hasn't moved in a few days. Guess there was still a little too much flow up on that rock.