Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Root tabs designed for garden use are good when the water level in the soil varies.
Water dissolves the root tabs a little at a time, and the fertilizer gets washed down through the root zone of the plant. When the soil is drier the root tab does not dissolve, so there is some remaining for the next time it rains, or the sprinklers come on.
This is not quite the way it works in an aquarium.
Root tabs for the garden, placed in an aquarium, dissolve in water. Deep under the substrate there is not a lot of water movement. When they start dissolving, but there is not much water movement, the water surrounding the tab gets rich in the various fertilizers, and the tab stops dissolving. Then the water movement (slight as it is) and the plant roots remove the fertilizers (slowly) and the tab continues dissolving. Even if the fertilizer moves upward though the substrate there are usually more roots that will take it in before it reaches the water column, or else the substrate can latch onto the fertilizer. (read about cationic exchange capacity). That works just fine. Plants get the benefit of the fertilizer, and minimal traces get into the water column.
If the tablet is too close to the surface of the substrate, too close to the water column, the fertilizer that dissolves is carried away by the water movement, and can enter the water column.