I had to dip one of mine 3 times before the pinched mantle was gone completely. I did it for 30 min about every two days ( not a set time, just when the rest of the mantle looked normal). The other clams I have that had a pinch only took 1 or two dips. I do have a new clam that wasn't opened up fully in the pet store, but is now, it was being picked on by a damsel. I had to move one of the clams in my tank because it was in a spot where my eyelash blennie liked to sit and wouldn't open up all the way because of the constant contact from the fish. hope something here helps

good luck