Water plants need reduced iron and oxidizing iron for metabolism. Iron ions will fluctuate with ph values. In an acidic environment. Reduced iron can exit easily. If  iron ions are combined with organic chelate , the iron ions will can become more stable in an alkaline environment. In aquariums,we usually add chelate ferrie to increase the utility of iron for water plants . therefore, ferrie level should be determined by the level of chelate ferrie.