Iron (Fe) is important micronutrient. There is often lack of this nutrient in aquarium water. You can observe it on the leaves of plants. The youngest leaves are yellow and whitish. It’s called chlorosis.
Iron is usually present in large quantities but mostly in non soluble forms which are useless for plants. Plants require iron in form of cations – Fe3+ and Fe2+ but these are present only in small quantities in water. Because of this fact, we have to supply iron in form of fertilizer.