When I had cory eggs I had to remove them or the parents ate them. The first 2 batche sI had hatch, I let them go when they were about 7 days old and no one them survived. Then I watched really close and I actually saw the adults chase down the babies and eat them. I was shocked. I would imagine that the cories are making snacks of the fry. As for the mystery snails, I have heard they are egg eaters so they could also the culprits. As for eating the fry, they would only eat them if they were dead or if they somehow caught them. With as slow as a snail can be a fry might not react until it was too late.