This is a hydro grow isn't it? ("halo dripper"). If so, get your ph to 5.5 and let it drift slowly up to no more than 6.2. If not, my apoligies, high 6's is perfect.

Pure ro water has no Potential of Hydrogen but the ph will change when exposed to air and not only will you get false readings with a ph pen it will damage the probe. Thats why a little ph up or down goes along way compared to tap.

Your tap water as it is will cause problems.

Subzero.