Citric acid is available in a pure granular form. A rate would be about 0.2 grams per gallon to remove 50 ppm alkalinity. Pre-mixed citric acid solutions (Seplex, GreenCare Fertilizer (815- 936-0096)) are also available for alkalinity control. Other organic acids like vinegar and lemon juice will also work, but because the concentration of acid in these materials is variable, for example, the acetic acid content in vinegar can range from 4% to 8% by weight, that the results that you get will not be consistent.