I had the same issue. I used flubendazole flake in a 55g, moved teh snails, and have been unable (for a year, so far) to move the snails back without them dying. I tried as recently as a month ago to house nerites in the tank. WIthin a day, they were totally inactive so I had to move them back out. Keep in mind, my tanks get 3x/week 50% water changes and do not have substrate so surface residuals should not be an issue this long afterwards. I did run purigen, but have not tried longterm solutions like poly filter 24/7 to try and neutralize the chemicals.