I think assassin snails are neat and want to keep them, but prefer not to feed them other snails. So what can they eat if live snails aren’t on the menu? They love any meaty food and frozen fish food works well as a diet. Mine love frozen mussels, with frozen bloodworms as a close second. They will also eat frozen mysis shrimp and in a pinch shrimp pellets but they would much rather have frozen food over freeze dried or flakes.
The picture above shows the snails congregating around a cube of mussels that was dropped in the water for them to eat.