Iron is an immobile nutrient in plants meaning that it cannot be scavenged from older leaves. This makes it very important to have a constant supply of available iron. Ferrum it a highly available form of ferrous chelated iron. Other cheaper iron supplements use iron in the ferric form which makes it less available. Dosing instructions are 1 pump per 10g 1-3x per week depending on the needs of your tank to maintain 0.1ppm iron.