Depending on how many pest snails you have, your assassins are probably living it up in the substrate. I raise them and they do die like everything does eventually but they are hardy and as long as there is food around you should not have to worry. But do not expect your assassins to totally eliminate your pest snails unless you toss in an army of them. One adult assassin eats about 1 pest snail every other day. Smaller assassins eat pest snails their own size. Adult assassins will bypass smaller pest snails on purpose or so it seems, to leave for their little ones. So you need assassins to breed and have a population of each size to really go after the pest snail problem. Until your assassins breed they will only eat adult pest snails. Once they breed and their young eat tiny pest snails you will notice the difference.
I have 19 tanks. I am now in the market for buying MTSs to feed my assassins, something I never thought would ever happen. It took 3 years but now I am almost out of MTSs and that is not a good thing either. BTW they will eat pond snails before they go after MTSs.
