If you really hate the snails... and you only have an oto in there. Take the oto out, and treat the tank for snails. Potassium permanganate or ammonia at 8ppm should kill the snails, without harming the plants. There is also Had-a-snail, but I have never used it and I don't know if it's plant safe.
Edit: Had a Snail will probably kill your plants.