Just one word of caution-while the behavior you mention can be perfectly normal for snails, it can also be a sign that they dont like the water quality. I wouldn't worry about it as your tank don't seem overcrowded, but sometimes a spike in Ammonia can cause large snails to skirt the perimeter of the Aquarium in an attempt to "escape". They then get caught between a rock and a hard place as they dont want to "dry out". It can also mean they're laying eggs or found a good food source on the waterline