well i directed my question to henry and this is what he said
"20 to 30 ppm NO3 will never kill a clam. They actually need NO3 to grow.
In clam farms, NO3 is deliberately introduced to feed them.
That's also the reason for some to have lots of clams, oysters etc in the hope that they could remove NO3.
The down side is, clams are ex and you never know when a virus attack it. Yours may be a case of virus attack.
Symptom - one day open nice and big as usual . .. then for no apparent reason, started shrivelling and clamp up leading to death."
my first clam died exactly in the way he said...i guess the virus is spreading so the cleaner go and nibble on the clam