LACTIC ACID
Lactic acid is a by-product of fermentation and is always present in trace amounts in any wine, beer or cider. Lactic acid is the safest acid to use for treating the water when making beer and will increase the acidity (lower pH).
As a guideline, ΒΌ teaspoon added to the standard mash will lower the pH (increase the acidity) by around 0.1