I’m having the exact same issue. And I *know* it is from the food on the bottom. The problem is I have otto’s and the have pretty much cleared all of the algae from the tank. So I’ve been dropping in an algae pellet every other day. Well the snails are always the first to those pellets, they eat the pellet up until there’s almost nothing left for the ottos. I’m thinking about taking the otto’s out and adding salt to my platy tank to try and kill the snails that way, or at least make them uncomfortable.
The platys are good about eating the food I feed them at the top, so its really the sinking algae pellets that are the issue. Over feeding makes the snails explode! I guess if you want to use sinking pellets / wafers you better be sure you don’t have any snails in the tank.