If the tank is stable, the ammonia the fish produce should be balanced by the nitrifying bacteria on the plants, the substrate, the filter media and everything else wet in the system, minus whatever ammonia the plants consume. In fact, if the tank is heavily planted, virtually all of the ammonia will be food for the plants, and the level in the water will always be too low to measure.
Similarly, the phosphates and nitrates in the water are valuable plant food, so much so that we have to dose even more to make sure there is enough for the plants. So, there is no point in trying to remove any of those compounds with filter media. The filter media, whatever it is, will be home to colonies of bacteria no matter what we want, and those colonies will have to be in equilibrium with the food supply available to them. So, about all that is left for the filter to do is remove fine particles from the water, keeping it very clean and clear. Filter floss and sponges do that job very well.
If a problem comes up, like excessive leaching of tannins from wood in the tank, that causes the water to become too discolored to please you, you can add charcoal to the filter to remove the tannins. Or, if you have to add a medication, such as Maracyn, to help get rid of cyanobacteria, for example, you can add charcoal to the filter to remove it later.
Purigen is another media that works well in a filter to do essentially the same job that charcoal does, but it is easy to "clean" so you can reuse it.
