Calcareous gravel, rock, coral and plant or algae growth can cause high pH. Tap water also affects pH. Fritz pH Lower is a liquid buffer that decreases pH & alkalinity in fresh, brackish, saltwater & ponds. Use it when topping off a tank, performing water changes or when pH is improperly balanced. It even works in hard, alkaline water! Many pH buffers won't work on well water because they don't affect alkalinity; hobbyists notice after adding other buffers that the pH returns to the original level within hours. This "pH rebound" is due to water hardness (from Calcium & Magnesium carbonates). Fritz pH Lower neutralizes the carbonates causing rebound so that pH adjustment is EASY. Quick and Easy Reference Reduces pH (increases the acidity) and reduces the alkalinity. Conditions water in freshwater, brackish and saltwater aquariums; works in reefs and ponds. Super Concentrated Professional Formula: Each capful treats 50 gallons in fresh & saltwater Info provided by Fritz