I've been following this thread for a while and no ones replied. 20 assassin snails might be difficult to come by. You could try AnchorBay aquarium in New Baltimore or Fantastic Fins in Lavonia. I've seen them at both places but not in that high of numbers.
I put 6 assassins in my 75 gallon. It was loaded with hundreds of MTS snails. Within a month or two there wasn't a MTS snail to be found. The assassins bred but not in high numbers. I don't think I could find 20 assassins in the tank if I tried.
Same thing with my 40 gallon. I put 4 assassins in and within weeks my pest snails were gone. They can annihilate a snail population in no time.
Your best bet might be to get a few and breed them.
