how much would be needed per gallon?
Personally my tap water is very base when I test pH it is very blue at around an 8. I add about 5ml per gal to bring it down into the 6’s. Now when adding nutrients it is a bit of an experiment. The GH trio runs pretty neutral from my experience, and since my starting water pH is really high I’m always adding lemon juice to bring it down. When I’m in flower, I use a lot of molasses which is acidic along with some other nutrients and my pH goes down to the 4’s, I use about 2.5ml of baking soda to bring it back up to the 6’s.
so start by figuring out your pH alone with no nutrients, then add Lemon to bring down or soda to bring up as a base line. Only has to happen one time not every time you feed since it will always be the same coming out of the tap.
then add your nutrients and check pH again and balance it one way or another. Going forward you will only have to pH when you add nutrients, since you already. Shoot for high 5’s or low 6’s