What other foods do you put in the tank?
I used to feed frozen bloodworms since they didn't seem like they were hunting very well - coincidentally enough, and it's surely just coincidence, I had assassin deaths only after I stopped using the worms.
Here is an interesting piece saying the the Great Pond Snail is a freshwater Whelk
http://books.google.com/books?id=b6...=X&oi=book_result&resnum=7&ct=result#PPA91,M1
I think the author is saying that the Great Pond Snail also goes by the name of Freshwater Whelk, and not that it is a freshwater whelk. Sort like how the Redtail Black Shark isn't actually a shark. I'm no expert on whelks, but I believe that name is just given to either a family or order - and one pond snails likely doesn't belong in.
I always see the Indonesia being the source of many exotic and undiscovered species. Tylomelania sp., Sulawesi shrimps such as Celebes beauty, and then the assassin snails.
Especially with the recent introductions to the hobby, their coloring seem much more akin to marine inverts than freshwater, doesn't it?