I've never had that issue with snails before, but maybe you should try to put bigger rocks around the roots or something to block the snails? Also, I would NOT switch from gravel to sand. Maybe if you're a more experienced tank keeper you can do it, but from my personal experience it is not good having sand substrate (at least as a beginner). When I first set up my tank, I used sand and it completely ruined my tank. The water was never clear and I was told it would kill my fish to keep the sand because it clogs up their respiratory system. In the end, I had to do a complete water change and scrub the whole tank out. Once again, if you have experience with this don't let me stop you; I just had issues with sand.