Is the shell eroding away??
shell detoriation: superficial wormholes; deep holes, loss of shell top
To have a good understanding about the causes of shell detoriation, one needs to know some basic shell anatomy [sread more about shell anatomy here
]. The most important thing to remember is that the rigidity of the shell is provided by a strong, calcified inside, with a protective protein layer at the outside. It's the latter that prevents the chemical detoriation of the calcium at the inside. Once the protective outer layer is damaged, the calcium layer is exposed to the water. This shouldn't be a big problem, as long as the water is rich in calcium and is not acid, but once the pH of the water drops and the water becomes acid (pH below 7), the calciums starts to dissolve. As long as this process advances at a slow speed, the snails is often able to enforce the calcium layer, although only at the inside. The ouside of the shell is dead material, and cannot be repaired by the snail itself, so once damaged, it will stay that way. The oldest parts of the shell (the shell top) and those places that are often hot when a snail fall on the bottom are also the places that are most vulnerable as the protective outer layer is often damaged at those parts. Problems arise once the shell is detoriated that much that holes are formed, exposing the soft tissues below. In case of large holes, the snail can get problems with keeping the mantle cavity
open, with lung
collapse and other problems as result. Nevertheless, smaller holes an pose a problem as well, especially in a crowded tank, as other snails and fish won't hesitate to eat the exposed tissues. Luckely, snails do have some kind of repair system: they simply calcify the exposed tissues to protect them.
So what to do once a snail has gaping holes and or a detoriated shell surface?
First of all, check the water quality: is the pH at 7 or more? (keep it between 7-8). How about the water hardness? (keep the kH and GH high).
A good way to regulate the water quality is to add a source of calcium in the form of crushed egg shells, specialized preparated, crushed sea-shells, marble or something similar. Once you are sure that the water is well enough to halt further detoriation, one has to decide if the shell should be repaired or not. If the snail is active, one can assume that the snail does not suffer from the damage. In such case a repair should be rather considerd a protective measurement to prevent other snails from attacking the exposed tissues. If however, there are no possible tissue eaters like fish and snails around, or if the holes are that small that the tissue stays out or reach, one can choose to leave the situation like it is. The snail will calcify the vulnerable tissues anyway as reaction to the exposition to water.