7.1 is nearly neutral. I wouldn't worry about lowering it at all in bark. Addition of most water soluble fertilizers will buffer toward a specific pH depending on the fertilizer and its intended use, and most organic media are naturally acidic as well. More important than the pH of your tap water is the buffering capacity, and a set amount of vinegar or citric acid can cause a much larger or smaller change depending on the quality of your water. The only way to know is to try, but as I said, I wouldn't worry about 7.1.
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