Chloramine/chlorine tap water should be fine. Not like the clam is going to be inhaling a bunch of tap water.
If you're worried, you could put the water in a bucket with some Prime.
I'll run frag plugs under tap water to get rid of aiptasia. They swell up and dissolve. I just try not to get water on the frags themselves.
Zoas don't care though. I'll run those under tap water without bothering to dechlor or buffer.