Good question but difficult answer.
I bought 10 adult assassins for my heavily infested with pond snails 30G tank. One year later, I still have the occasional pond snail & the assassins have multiplied profusely.
I have seen it suggested that the assassins have their own tank and you transfer the pest snails to that tank. I found that a pain so I just remove the assassins as they multiply and sell them at my local fish club meetings.
I do have assassins in tanks with ghost shrimp and Red Cherry shrimp but have not noticed any predation of the shrimp, maybe because of the large number of pond snails.
BTW, I am using very fine sand in all my tanks. The assassins enjoy burying themselves and assist in stirring up the sand. I harvest my excess assassins using a small kitchen strainer to scoop up the sand & sift out the snails. This method works great for me.
Hope this helps.
