One way to control mystery snail populations is to remove them from the water when they become too numerous. This can be done by hand, using a net, or by using a trap. Another way is to control mystery snail populations by changing the water conditions so that they are not conducive to mystery snail reproduction. This can be done by raising the temperature of the water, or by adding chemicals to the water that will kill mystery snails. Lastly, you can control mystery snail populations by simply removing the food that they eat. This can be done by removing aquatic plants from the water, or by changing the type of fish food that is used.