Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Well, it is going to depend on how much you put in there.
Remember that garden fertilizers are designed to work in a relatively dry setting. When you irrigate, some fertilizer is released. Then the soil dries out and no fertilizer is released. The plants use the ferts made available when you irrigated. Then you irrigate again, releasing more fertilizer.
In an aquarium, especially in the main water flow, the fertilizer is exposed to "irrigation" conditions constantly, so is constantly releasing fertilizer. This is fine if you want water column dosing, and add only a few pellets at any one time.
Part of the idea of using slow release ferts is to get the tablets (or little round balls) deep under the substrate so there is less fertilizer in the water column. Some ferts will trickle up anyway, so there is a combination of substrate dosing and water column dosing. If you set up the fertilizer in the filter you are simply water column dosing.