About 5 weeks ago, I added an Assassin snail to my daughter's 10 gallon tank to help with pest snails that came in on a plant. Everything seemed to be going well for about 3 weeks. Fewer pest snails, very active assassin snail. For the past 2 weeks, I have not seen any trace of it and the pest snails are getting larger (in size) than ever. I have dug around in the gravel (I know- sand is better for them) with no success in finding it, not even finding its shell. It was able to burrow about halfway into the gravel, with the driftwood over the top of it. The other occupants of the tank are Platys, Aspidoras Spilotus catfish, and a Flying Fox. When doing water changes and cleaning the tank, I add Seachem Neutral Regulator 7.0, Stability, and Stress Coat. Is something I add during water changes harmful (could something cause a shell to dissolve - nothing has affected the pest snails)? Or could one of the fish killed it or might it have climbed out? The only openings it could get out are where there are cut outs in the lid for the filter and feeding access. I never saw it on the glass of the tank any time I saw it during the first 3 weeks before it went missing.