those aren't the only elements and trace minerals that our critters use:wink: .....stuff like strontium, potassium, iodine and a host of others will slowly get used up.....if your not adding these things and not doing regular water changes, then these minerals and elements will never be replenished....

So how big of a water change do I need to make an impact. I've got 250 gallons total.

actually i just remembered. i did a 10 gallon water change about a week and half ago. few days after i noticed the clams close up. it didnt change anything.

also, this wouldnt really explain why they would open slightly more at night

So you wait 6 months, then do a water change, then all of a sudden clams start acting up. Stability sounds like an issue unless there's something else you've forgotten about?

no. i wait 6 months, clams start acting up. i do a 10 gallon water change. no change except at night (which was happening before and after the minor waterchange)

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)

is it possible it was the moorish idol since it started weeks ago and he only died like a week ago or so??? theyve been known to pick at clams, i would still go with maries idea with the basket and since your a photographer is it possible to video the tank to catch the culprit maybe in the nighttime??:)

If your nitrates are at 0 that is one problem. Clams do need some ammonia/nitrate to fuel their zoozanthellae.

Also check your alk, low alk can cause those symptoms.

Another option is pyramidellid snails, check in the scutes for them and under the clam, around the byssal opening. The snails are generally nocturnal unless there is a heavy infestation.

Clams from Vietnam are also not the healthiest. They tend to carry perkinsus aka pinched mantle disease.

Cleaner wrasses are also known to go after the iridophores on clams. My suspicion is it's a combination of the cleaner wrasse and no nitrate for food.

Cheers,
Tim

Can't find a cover for the clams with holes that the cleaner wrasse can't swim through

Local pet stores have mesh breeder boxes that suction to th glass. If you can move the clams. Put them in there at top of tank