If you do use Osmocote make sure you do it right, don't just mix it into the substrate.
I made a noob mistake and mixed Osmocote into my substrate, and some of the granules were left at or very near the surface. This caused 2 problems.
The first was that ammonia skyrocketed out of control for several days and killed some of my fish. Even doing daily water changes, a couple got sick and died. After a few days the ammonia levels stabilized. I don't know if it is because the bacteria and/or plants adjusted to consume more ammonia or if the osmocote just releases its fertilizer way faster in the first few days. I think it's the latter, since I have read other people say the same thing, that it releases about 50% of its total content within the first 72 hours or so and the rest is released slower.
The second is that over time, the granules become buoyant somehow and can float around. I was picking Osmocote spheres out of the foiliage of my anacharis and other plants constantly. Very annoying.
P.S. - Just something I found interesting, I remember someone on these forums was saying that the Osmocote spheres are capsules that contain liquid fertilizer. This is not true, but I understand how they might get that wrong impression. The Osmocote beads are actually solid, dry fertilizer, coated with whatever coating they use to make them slow-release. If you break one open, it's hard inside like candy or something. However, after they have been in your tank for a while, the water penetrates the inside and eventually dissolves the solid fertilizer stuff inside, so if you pick up a bead that has been in your tank for a while and smash it, the thin outer "shell" will pop and you'll see the inside will be filled with white liquid. They don't start out liquid inside though. Just wanted to clear that up
