By no means am I an expert on clams. I got my first two from Barry at Christmas time, and within a couple of weeks I noticed the signs of PM.
It started on one but within a couple of weeks both were showing signs. They recommended the fresh water dip so I did and everything seemed OK.
Here's the thing, both clams open right back up within 10-15 minuets and look perfect. But, within a couple of weeks, I see the signs start to appear again. I was hesitant to do the dip again but I did, and they again looked great.
I kid you not, I've been doing this since mid January, probably at least 8 times each. It doesn't seem to hurt them at all. I assume because I don't wait until they look really bad, they are healthy enough to go through the stress without a problem.
I add about 1/4 teaspoon of baking soda to about 3/4 gal of RODI water and warm it up to match the tank and plop they go.
Now you may ask if I'm sure they are actually suffering a case of PM. Well, I do usually wait about a week to see if they uncurl by themselves, but after a week they are really looking curled up. I don't let it go any further and so I guess they are still technically pretty healthy.
I don't know if this will help anyone, but I wanted to share my experiences. Good luck.