Depends upon your grow method... If you're in soil, like me, it may not be necessary. I use distilled water, so I know that should not be an issue for ph. I'm about to harvest my very first grow and my two girls are 35 inches tall and look to be holding about 2ozs each. I also have never checked ph, and am considering a switch to filtered tap water in my next grow. I may check the tap water ph, but only to be sure it reads close to the distilled I've been using. Even at just $1 per gallon, the distilled water cost is adding up when the plants get big enough to suck it up...