Does anybody know of some household items like lemon juice or something along those lines that can serve as a pH up or a pH down solution?
I think it's much better to just buy some PH up and PH down. I guess in an emergency you could try to MacGyver something from household chemicals, but why not just use the inexpensive products you could buy that were made for the purpose?
To lower ph in soil: Phosphoric acid, vinegar, lemon juice, sawdust, composted leaves, wood chips, cottonseed meal, leaf mold, peat moss, or PH Down. To raise pH in soil: potassium hydroxide, dolomite lime, hardwood ash, bone meal, crushed marble, crushed oyster shells, or PH Up. I cannot vouch for what any of these would do to a hydro system.
No! A) It's a salt and is NOT good for plants and B) It'll mess up your TDS measurements. Learned this the hard way on my own grow. You're better off just getting some pH Up & Down from the local grow store. They work well. Their effects are more predictable (instructions on the bottle). And they're made of stuff that plants like (potassium hydroxide and phosphoric acid).