I have 2 Yellow-Horned Nerite Snails in my fish tank with 2 other goldfish. They just eat the algae and do their own thing. Then, today I found one of them upside down. I looked into it and show that there appeared to be nothing inside of the shell. I smelled the shell to check if it had died and there was no bad smell. It just smelled like the tank water. I am very worried but I am sure that it had died. Some background: A couple days prior it had been in one general spot on the wall and never moved much. Can Yellow-Horned Nerite Snails leave their shells? Does it need a bigger shell? Maybe one of the goldfish ate it? Please let me know what you think. I inserted some photos of the shell below. Thank You.