I have high PH water that's 7.8 - 8.0
I use regular ole cheap white
Vineger. A tablespoon in a gallon of water
brings it down to 6.8 I let that sit for an hour or 2 then check again and find it to be around 7 so I add a 1/4 - 1/2 more tablespoon and gets right around 6.5 and then I mix in my nutes.
Careful with other people's suggestions on what to use. I figure some people just like to type big words, name the highest priced stuff and/or tell you something works when in fact they never tried it themselves
