Why distilled water?

If you mix the iron solution with the phosphate solution the iron combines with the phosphate to form a precipitate - clouds the water temporarily until the precipitate settles on the substrate. If you do this in a premix of all ferts it is serious, because none of the fert doses will contain the expected amount of iron and phosphate. If you do it by dumping the dry powder into the tank, it just temporarily removes those ferts from being available to the plants. Eventually the precipitate will be consumed as a substrate fertilizer. So, those who sell ferts always say don't dose both at once, and don't try to mix up all of the daily ferts in one bottle.