If you keep shell dwellers, you probably have a large number of shells in your tank(s), especially if the tank(s) contains multiple dwellers. That’s a good thing and a bad thing.
Among other reasons, providing multiple shells is often conducive to successful breeding colonies, so a large number of shells is good. However, shells are notorious for collecting detritus. Though shellie cichlids are good house keepers, unoccupied shells are left to do nothing but collect bad stuff, which can get lodged in the shell where you can’t easily see it. As the amount of waste accumulates, you might notice a rise in your nitrate concentration, especially if you have numerous shells that are unoccupied.
Many species of shell dwelling cichlids will bury nearby shells that they aren’t using or otherwise fill them with sand. Thus, shells that have been covered and buried, you know they aren’t occupied. However, don’t conclude that, because they haven’t been filled with sand, that they are unoccupied. Also, you shouldn’t conclude that, if they have been filled with sand or buried that they don’t contain detritus. Fish won’t bother cleaning out shells they aren’t occupying before filling them with sand or burying them. Therefore any garbage already in the shell gets sealed inside.
Knowing which shells to clean can be a difficult proposition, especially if you have multiple shells that are occupied. This is particularly true with females who may be guarding eggs. Also, some very small species can lodge themselves deep in shells where you can’t see them. Unless you’re certain a shell is unoccupied, unless you can see eggs, or until any fry present themselves, you should be quite careful. You might unnecessarily dump contents out of a shell that you shouldn’t have.