I agree with post 16. Aquarists sometimes forget (or may not realize) that these small snails will reproduce to the level according to their available food. If they have no food, they cannot survive. So if you see dozens or hundreds, it means they are eating organics. Fish excrement, dead/decaying plant matter, uneaten fish food...this is all food for snails. And it is beneficial they eat it. But the point is that the supply of food will determine their numbers. Major water changes including good cleaning into the substrate and keeping the filter well cleaned will aid in removing their food.