My snail Kacper hasn't moved for three days and i just cleaned my tank because they usually become inactive so i just clean my tank. Even though i clean out my tank every saturday. Well i took Kacper out and i smelled him, Kacper didn't smell like awfully bad like you guys have described but so then i had of touched his trap door which hadn't of fall out but this black substance came out, like a layer of skin. It was weird so i put him in a cup by himself with blood worms and i poked his trap door once more and that black substance came out, i haven't read anything that had to do with this black liquid coming from the shell. I believe he's dead but what would you think of this?