I would cut it up actually, and refrigerate it, and throw pieces in. The trapdoors are good, and I would just let them breed in the darter tank. The newly delivered? trapdoors are about as big as a darter can kill. Not sure that they can with the door. Might always have to smash these. What you want to cultivate are pond snails. The small ones that you find in almost any body of water. The trapdoors will do no harm in either place.
Wait you're saying that trapdoor snails can kill my darters?