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.
Average planted aquarium needs aproximately 0.5 ppm of Fe a week. You can add it in form of liquid fertilizer (Seachem, Tropica etc.). But this solution is quite expensive so you can add dry fertilizer instead. It’s much more cheaper and results are the same. You want to use Tenso Cocktail or CSM+B as a source of iron and other micronutrients.
25 gallon tank needs weekly roughly 1/3 tsp of Tenso Cocktail.