Quote Originally Posted by Abyss
You NEED a PPM/PH pen to monitor your nute levels and PH. This is an essential tool if you want success with in this growing style. Get some PH up and PH down from your grow shop if you don't have some already.
Absolutely essential. Each are around $20-$30. The way you tell if you have enough food in your solution is with a PPM meter. Of course a PH pen is even more important. You can kinda guess at PPM but not PH.

And if you have a 40 gallon res, keep in mind you probably will not be filling it to the brim, maybe like 38 gal.