I added my clownfish in the other day and he was looking a little sickly so I dropped in some Melafix as it clearly showed a SW fish on the bottle and said it was safe for all fish. Nowhere on the bottle did it mention having copper in it.
My 3 snails ALL almost immediately dropped off the rocks or glass and pulled in TIGHT. They would start to poke out again and clam back up. The hermit crabs seemed unaffected. I was worried about the snails and had an extra carbon filter from my big tank so I put it into the filter to get the Melafix out in case it was toxic for snails (never occurred to me that it might be b/c I have mystery snails in my FW tank and it never harmed them).......
This morning, 2 snails are back up and at 'em, no problem, but my astraea hasn't moved since last night and I'm pretty sure it's dead.
All told though, I'd rather lose a $2 snail than a $20 clownfish!