What sort of plants are in the tank?
Post a picture of the tank so we can see how much light is in it and how many plants you have.
What are the tank dimensions?
What sort of lights are above the tank?
Do they have red, green, blue, etc, light?
I used Sera Florena. It is an iron based aquarium plant fertiliser. I used a Sera iron (Fe) test kit to monitor the iron levels and keep them at 1mg/l (1ppm).
You might not need to add fertiliser (depending on the plants), or your light might not be bright enough to warrant adding fertiliser. If there is good light and lots of fast growing true aquatic plants, then you add fertiliser.