Don't let the name fool you, assassin snails are not "bad" snails. Instead, they are actually tough, useful snails that will eradicate your pond or aquarium of obnoxious snails, such as Ramshorns, Trumpets, and bothersome Pond Snails. They may look innocent during the day, but by nightfall, they're as diligent as they come.
It's important to know that assassin snails do not typically attack larger snails, preferring instead to go after baby or weaker snails. They will not eat other assassins, and while they may go after a shrimp, it's more likely they avoid them.
Assassin Snails are especially loved by owners that have Malaysian Trumpet Snails that could have 100+ babies or so at at time. Simply bring in the assassin snail, and they will take care of the problem for you.
When out of food, they will take care of waste in the substrate for you, so don't worry too much if there is not much food for them!
In the aquarium, use 1 for every 3 gallons of water
In the pond, use 1 for every surface foot of pond area