Exotic, dont waste your money on that stuff. Almost anything, selcon, zoecon, vitachem, zoe, or frozen cyclopeeze will get them to open with just a few drops in the water.
I would agree also to wait until you can find a piece without any recession. About 50 times i almost bought one that only had a "little" recession but kept waiting. Finally found a REALLy nice piece with no recession. Definitely worth the wait!