Here, M8. Check these guys out...
These are hydroguys, but AG seems to know his stuff with vinegar and ph
I came across a thread saying they use different vinegars as a down and baking soda as an up. So I added a cap full of red wine vinegar to about 3 gallon (maybe a little more) res and it plummeted to like a 4.3. I added ph up to bout 8 oz water and filled the res, cycled the system, got it to 5.3, then repeated and nailed it to 5.8 so the kids were only on the acid for maybe 10 minutes.
Damn, a full cap of vinegar is comparable to 2 teaspoons at the least; I have only added .3125ml (1/16th Tsp) to my 1 gallon of nutrients lowering my pH from the high 7.8s to right around 5.8-6.0. But as we both know using vinegar doesn't stabilize pH for an extended period of time. Maybe this is due to the vinegar evaporating, I don't know? I'm in desperate need of some hydro labeled pH buffers. That at least would be optimal for my system to run without my presence.
One other tip from one grower to another, I tend to mix a small cup full of water and add my amounts of vinegar to concentrate that solution with an acidic pH. Then I usually can get more leeway when adjusting pH by just pouring small amounts at a time to the concentrate. But like you said if you get to low, just add the original water source.
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These are hydroguys, but AG seems to know his stuff with vinegar and ph