I wouldn't recommend battery acid as it is contaminated with all sorts of heavy metals etc. Doesn't have to be very pure to be fine for batteries.
I've always used concentrated sulphuric acid to drop my pH in my tubs in the past. 4 or 5 drops would knock it down to 5.4 from 6.2 in a 50L tub. Then when the tub needed topping up with RO a few days later it would be back up to around 6.2. Always only ever used RO water so didn't have to deal with carbonates and other crap in my water. I used to be a chemist in a hazardous waste facility and brought home quite a few things from there. Still have most of a 9lb bottle of 95.9% analytical grade sulphuric acid and a quart size bottle of nitric acid of the same strength. Made my own calcium nitrate with it for fun using lab grade marble chips. lol
Bubbling any water is a good thing to do whether doing hydro or soil growing. Charges the water good with oxygen before it's used and gets the chlorine out at least. For soil grows it's really a good idea as the O2 content of tap or RO water is no where's near as saturated with O2 as it could be. Often wondered it running pure O2 from say a welding tank would give noticeable results because when you bubble air thru the water you are also adding nitrogen gas which is about 78% of the air we breathe.
I went back to school in my early 30s and spent 3 years getting a diploma in environmental chemistry. Never could make much money doing that stuff but it sure comes in handy when messing with nutes etc. Built my own still so I can recover the solvents I use to make oils. Hard to get things like ISO or naphtha here so I can really stretch out what I can get. ISO is about $25/gal and even the Coleman Camp Stove fuel I buy to distill for the naphtha is $20 a can. Using a couple of flat-bottom boiling balls and some copper tubing I've even distilled my 9x filtered butane so it can be reused. That stuff ain't cheap either.
Without that diploma and all the industrial training I've had in hazardous waste disposal I'm sure I would have blown myself up by now.