CSM+B has iron in it, but if you want more than that you can add chelated iron. Not everyone agrees on how much iron is enough, let alone ideal.
With 2 watts per gallon of light your plants will consume quite a lot of nitrate in growing. 10 ppm is barely adequate, even if you were maintaining that amount with every other day fertilizing. Those two facts (as I see them) say you have to dose nitrates to have good plant growth and inhibited algae growth. KNO3 is also a major source of potassium (K), so you need to dose for that too, either with KNO3 or K2SO4, so why not kill two birds with one stone and dose KNO3?
Incidentally, your test kit may say you have 10 ppm of nitrates in your tap water, but those kits are not known to be very accurate. That 10 may actually be 5 or 15, unless you calibrate the kit with a known ppm sample. And making known ppm samples can be very difficult to do accurately. Still another reason to use EI fertilizing.