Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Like many other answers in the aquarium hobby, it depends.
Coarse substrate allows more water movement, and a significant amount of the nutrients will end up in the water column. Test for ammonia, nitrite and nitrate. If you have tests for phosphate, iron or any other plant nutrients these would be good to test, also. But I would not go out and buy them just for this.
Finer substrate, and pressing the fertilizer all the way to the bottom minimizes the amount of nutrients that get into the water column.
If you have floating plants, or plants anchored to wood or rocks, then they will need some water column dosing from the substrate tablets, or from other methods. If all your plants are rooted you can greatly reduce or omit water column dosing.
Osmocote will release more than just N. If you see N in any form in the water, and you know it is from the Osmocote, then you can assume that all the nutrients are getting into the water in roughly the same % as in the fertilizer tablets. In other words, if you think you need less N, you also need to cut back on P, K, secondary and trace minerals.