Whats wrong with my Clams?

[eli@fijireefrock.com]

From reading all the posts in this thread and seeing that everything seems in check (but still think you need to make a bigger and more often water changes as you said 10glns for a 240glns tank meaning 0.05% total volume)
In my opinion and past experience I think some or one of your fish has acquired a taste for your clams as you mentioned that they seem to open up at night when fish are not around (as some folks already mentioned)