Usually the packets you get with the pens will be a calibration powder that you mix with water in a small container to cal your pens, not sure if it will be strong enough to PH nutrients in a large res, would be much better off getting yourself a good PH down, i use growth technology PH Down which is phosphoric acid, beware of flairform as it is citric acid based and will break down a lot quicker, id only use it in organic