Get some Bayer. I had monti eating nudi's. Took me about 2 months of dipping 3-5 times a week to get rid of them. I also keep all my monti's on the sand bed.
12 ml to a cup of tank water. 15 minutes and then through two more containers of tank water to rinse off all the bayer. Be sure to rinse it off completely before putting back in your tank.