Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you use something small like Osmocote you can push them in, but your grid may not be perfect. That is OK, though. By the time the planting is so dense that it makes the placement tricky, the roots are all over the place.
The life span of plant tablets is variable.
Slow release tablets dissolve slowly. (duh) but only if the substrate is fairly dense, with just a little water movement.
If the tablet is in a gravel substrate then the water movement will wash away the fertilizer as it dissolves, so more of it dissolves faster. You can check this by testing the water. Whatever nutrients are in the tablet will show up, for example, NO3 test would rise, if there is NO3 in the tablet.
If the tablet is in a substrate with a high cationic exchange capacity then the nearby substrate will get saturated with the minerals, and act like a reserve, even after the tablet itself is gone. Plants can take the minerals away from the substrate, so at some point the minerals do get used up, but hopefully you have added more ferts before it becomes a deficiency problem.