I ALWAYS give my snails a H2O2 bath before putting them in my tank.
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Same thing just happened to me, and I see the Nassarius snail spreading a mucus sticky trail all around that looks and feels exactly like the stuff that killed all my fish. I think they may be carriers and have started researching it. I treated the tank with Pimafix as instructed on the bottle for my tank size. The last addition of a fish was a Bengai Cardinal which is still alive. I originally thought the Bengai brought the velvet as it was the most recent addition and I noticed it had a suspicious "hole in the head" like white-ish spot on its head, but the velvet would need time to overcome the fish. This morning I watched the same snail spread slime trail around the tank and even sand gets stuck in this viscous gelatinous gunk.Got the snails a few weeks ago from a very reputable LFS in the area, just lost everything over the last few days.