Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Osmocote is made for gardens. It has a covering that allows the fertilizer to seep out slowly when plants are watered. As the soil dries out the fertilizer stops seeping out. Then the plant is watered again...
In an aquarium you can put Osmocote directly into the substrate. It must be buried. The slower water movement under the substrate will slow down the release of the Osmocote. When all the fertilizer is gone there is still a thin, hollow shell that pops if you pinch it.
If the Osmocote is too close to the surface, or you place it in a glass of water all the fertilizer is released fairly fast, but the shell still remains.
In an aquarium this can lead to an ammonia spike.
People use the capsules to make it easier to put the Osmocote deep under the substrate. They are optional, though highly useful.
A fertilizer that fully dissolves in just a few hours is not going to be slow release in the tank, even with low water movement. Test the water the fertilizer dissolved in. Any ammonia? This would be bad for the tank.