Whats wrong with my Clams?

[MarkoD]

Suggestions would be welcome if someone knew what the issue was.
I was expecting someone to know why they're closed and then offer a solution.

Not offer 5 different solutions to be done at once.

So I'm going to do them one at a time and first I'm going to try and rule out a fish picking at it.

I just find it hard to believe that these clams are fine for 6+ months and then one day they just decide its no longer good enough at the same time. While nothing else in the tank is effected