One of your shellies gone missing? Assuming there is no way for it to escape the tank and there are no rocks or any ornaments it could be stuck under, check your shells. Depending on the type of shells you use, the fish may have gotten lodged in the shell and is unable to get out. Also, sick or injured shellies will sometimes wedge themselves in a shell to avoid aggression and/or harassment.
If you have lots of shells, how can you possibly know which shell a missing fish might be in? There are a few signs you can look for.
If you have a sand substrate and you’re pretty certain the fish is dead, start with the shells filled with sand. Unless you have too many shells to keep track of, find the shells that have recently been filled with sand. Shellies will often bury empty shells as a territorial behavior as to not allow competing shellies to take up residence nearby. However, a dead and decaying fish is a nuisance to all of the tank inhabitants.
I recently had a female L. caudopunctatus go missing for several days. After not seeing her for more than four days, I knew she must be dead and probably in one of the shells. Her tank contains less than a dozen muffin snail shells and, because I pay close attention to my tanks, I am familiar with every shell. On about the 7th day of her absence, I noticed one particular shell that had just become filled with sand. Sure enough, I dumped the sand into the tank and this is where the second sign comes in.
Smell. If you’re convinced a fish is dead in a shell but you don’t know which one, remove each shell you know is unoccupied and smell the aperture. If it’s been at least a week, you’ll know instantly which shell it is, even if you can’t see the shellie’s caudal fin inside (they’ll lodge themselves head first). The smell is unmistakable and a dead giveaway, pun intended. I pulled the sand-filled shell mentioned above out of the water, gave it a quick sniff, and…yep. I took the shell to the sink, gave it a quick shake, and out she came. I would actually recommend that you do this outside. Once the fish comes out of the shell, the odor will linger in any room you’re in…and it is quite unpleasant.
There is a third sign to look for: white, stringy substance flowing out of a shell. This is a sure sign a fish is dead in the shell. After several days of decay and if the shell hasn’t been filled with sand by another shellie, you’ll notice white billows of string around the aperture.