Snails (the small harmless ones as shown here) are your friends. They are very useful when it comes to breaking down organic matter which allows the various bacteria to act upon it more quickly. These snails eat algae (that you/we cannot even see), excess food if any, dead plant/animal matter, etc. This is all beneficial and part of a healthy ecosystem.
The snails will reproduce, according to the amount of available food in the tank. If this food cannot support the number of snails, they will not reproduce. So the number of snails you see is in relation to the available food they have, and the goal is to get that food out of the system as fast as one can, and the nails help with this.
Most of us have no idea how much organic matter is actually present, so these snails reproducing rapidly is a surprise. But they help in maintaining a balanced biological system, in the aquarium just as snails do in nature.
As for controlling them regardless of the above, I do not recommend Assassin Snails. If one of these should happen to get into your local ecosystem, they can reproduce and decimate the native snail species, which is never good. Chemicals like Coppersafe should never be used in a tank with fish or plants as this does impact both. And fish that will eat snails are not usually a good idea unless you have the tank space and setup suitable for the needs of the fish.
