Derasa clam, pinched mantle?

Hopefully someone out there can help me! I just bought a clam which I believe is a derasa clam just 4 days ago. Since I had it I noticed his mantle has inward curls. I think it's pinched mantle syndrome. Can someone confirm? And I read freshwater dip is the cure. If it is pinched can I use distilled water? Add ph buffer to it? Don't wanna lose my first clam. Ty