Help - Crocea clams not happy

[Ruth]

I don't know what is going on. I have 3 crocea clams, 2 squamosa's and a derasa clam. All 3 crocea clams have their mantles pulled in and are not extending. The other 3 clams appear to be fine. It started last night - or at least that is when I noticed it. I cannot see any predatory snails around their mantles. Water quality seems to be fine.
temp - 79
Ca. - 460
Alk/Dkh - 9.2
Mg. - 1350
Nitrates 10
Nitrites - 0
Ammonia - 0

My Nitrates always seem to run about 10-15ppm because of my fish load. I have had these clams for about a year.
I added a couple of acros and an acan last week.
The only thing that has changed in this tank in the past 8 months is that I was feeding reef roids for a couple of weeks but stopped last week when I noticed that my green carpet anemone was not doing well.

I was going to try a fresh water dip but am worried about stressing them some more. Any suggestions?