Snail spawns on the a leaf 
Baby snail's size about 2mm 
Teenager snail size about 6mm 
Turning to adult, size is bigger approx. 1cm 
This is serious! Shell detoriation of an adult snail.
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 calcium 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 outside 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. Luckily, snails do have some kind of repair system: they simply calcify the exposed tissues to protect them.
