Perhaps you should try adding your convicts to that tank. I know my Bartoni obliterated the 1 million (ok, probably an exageration) snails that were infesting my sump and my rearing tank.
All I found were empty shells after a week, and believe you me, there were TONS of them. In all my years keeping fish, and cichlids in particular, I have never kept convicts, but I would guess they will dispatch your snails rather quickly.
If your snails are going under the substrate, then you better get some loaches. They will find them there without a problem.