[medicineman]

@frozenbarb,
Perhaps the snails are just looking for extra nutrients.
Other than the N-P-K content, the tablet also consist of minerals like calcium. Snails probably just in search of those extra things they need, to mention one is material uptake for strengthening their shells.

@kayakbabe,
Too bad the tank was somehow neglected. Though I'm glad to find that the tabs work well and somehow it helped the plant to endure hard times better.