Fish waste and the natural process of aquatic plants will take care of most of the microsystem's nutrient needs, although regular water testing is reccommended in case anything is thrown out of balance. Some nutrients are harder for plants to absorb through plant food, like iron, especially if the pH of your system is high. This can be solved with additives like iron chelate, or monitoring the overall pH of the system.
Plants that absorb too many nutrients for their purposes will also have health troubles, known as nutrient toxicity. This can be caused by overfeeding your plants, either through additives or your fish's natural processes. If you see plant's leaves turning a dark green, or black color, it may be time to remove some fish; your plants are getting too many nutrients.
