Micronutrients- Iron

Plants, like humans, need Iron to move oxygen through their systems. Iron is necessary to help produce chlorophyll and is also an essential tool for some enzyme functions.
 
Plants grown outdoors naturally get their uptake of Iron from the soil. As is the case for all micronutrients, the  tiny levels of Iron required are usually available from the breakdown of older natural materials such as plant matter. Plant roots can also absorb Iron from Ferric oxide, a chemical present in the soil that gives dirt a distinctive red colour. 
 
When growing indoors, plants receive their dose of Iron from the nutrient fertiliser you are using. Plants take up Iron in its oxidised forms Fe+2  Ferrous ion or Fe+3 Ferric ion at around 1-5ppm.
 
Iron deficiency shows itself in sickly yellow leaves, especially in new growth. However, many micronutrient deficiencies look similar, so it can be hard to be 100% sure what it is that your plants are suffering from.
 
Causes of iron deficiency vary, but the most common is the form of Iron that is in your medium. Iron is more soluble to plants in Fe+2 form. 
It can quickly react with other elements in the soil to oxidise into Fe+3, which is much less soluble, especially in neutral or high pH soils. 

I would advise checking that the pH of your soil is not exceeding 6.5 if it decreases the pH by adding fertiliser with higher acidity. This may take a few weeks to see results, however. Try a chelated iron fertiliser that keeps the Iron in a soluble form to take quicker action. 
 
 Iron toxicity is rare, but an excess amount could lead to bronzing or tiny brown spots on the leaf surface. Some plants are relatively resistant to high iron levels. Still, it can affect the plants' phosphorous uptake, leading to other problems. Toxicity can occur due to a too low pH within the medium. Iron is more readily available in lower pH medium, and so at these levels, the plant may take up too much.

 It could also result from an excessive application of Iron, most probably when attempting to correct another micronutrient deficiency. If the pH of your medium is too low, try flushing your plants with clean plain pH adjusted water for 1 or 2 days. Start your nutrient feed again but maybe with a slightly lower dosage when you have done this.