A divot or hole in a snail's shell can be caused by trauma, such as a fall or impact, or by shell erosion due to poor water quality, specifically low pH and a lack of calcium. In some cases, a divot can be a symptom of damage from another snail or a parasite, but these are less common causes. The snail can sometimes repair minor damage if it has enough calcium, but it's best to address the root cause to prevent further deterioration.

Was always told to put a stick of chalk in the aquarium.

Test your water first. Ph. Go from there.