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You can get Sulfuric Acid at AutoZone or Nappa. Trust me bro this shit doesnt Fork around! Its very sensitive and very stable. Get you an eye dropper of some sorts to administer. I use 10 gal of water in my res and if I want to adjust the Ph by a point or 2 I just add litterally 3 to 5 drops. The Ph doesnt fluxuate but very subtle maybe a point upwards in a 24 hr period. I contribute this to the uptake of nutes and water hydrating from my girls thirst. If you check on your girls daily (which you should) then you make the neccessary adjustments for Ph if needed and top off the res every other day. I use advanced nutrients 3 part with big bud bloom enhancer and my shit is tizzzzzite. Be looking for my post in a thread MisterE started on Alien NLX as I am growing the same strain. I will post a pic on day 40. It should be nice!
Seriously Sulfuric Acid go get it! The shit rocks, use caution and be safe it isnt anything you want to play around with!