Iron is immobile in plants. This means that plants cannot divert iron from older leaves to new ones. Therefore, deficiency symptoms appear first on new or young leaves. Because plants use iron to produce chlorophyll, a lack of iron results in chlorosis, or yellowing, of the younger leaves. Stems may also appear short and slender. If the deficiency is severe and prolonged, each new leaf emerges lighter in color than the preceding leaf.
When choosing an iron supplement, it is important to know the distinction between the two forms of iron. The iron will be in one of two oxidation states: ferrous having a +2 charge, or ferric having a +3 charge. Ferrous iron, the preferred iron form and is soluble in water at any pH. Ferric iron, however, is only soluble below a pH of around 5.5; but if the pH is higher than 5.5, which more than likely it will be in a planted aquarium, the ferric iron will become insoluble and precipitate, settling in the root zone. Once this occurs, foliar absorbtion becomes impossible.
