Aqua One pH Down assists in the adjustments to pH in your aquarium. pH is the measure of alkalinity and acidity within a liquid. pH measured on a scale from 0 14 with 7 being neutral, 0-6.9 is acid, 7.1 14 is alkaline. Each change in one pH unit reflects a tenfold increase or decrease in the pH value e.g. pH 10 is ten times more alkaline than pH 9 and one hundred times more than pH 8. It is therefore extremely important to makeadjustments to pH slowly.