Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I wouldn't let them sit on top of the substrate. In contact with the water column where the water is moving more, I think they would dissolve way too fast, and the clay would cloud the water.
Pressed deep into the substrate where there is very little water movement they will not dissolve so fast, and the fertilizer will not get into the water column so much.
Part of the idea of using tablets is infrequent fertilizing. Press the tablets down into the substrate maybe once a month or so, not drop them on top like lawn fertilizer.