It's also said to leave complete dark the last 3 days. No truth or proof to either one. As for the spa ph reaper is right , much more chem's than you want. I have been around pools and maintanace enough to know. You can get ph up and down where you get the nutes. If not the comercial greenhouses use sulfuric acid, that's right battery acid and it works good but just a drop goes a long way. You can get at auto shops.

The lucas formula is 1 part micro 2 parts bloom. Then adjust the ppm's for the stage of the plants life. No grow in the lucas and that would make 5-10-9. Easy to follow and no need for gro part. The nutes have ph buffers in then for stability.

FloraGro
2-1-6
FloraBloom
0-5-4
FloraMicro
5-0-1