Terra Aquatica pH- uses nitric and phosphoric acids to do the initial heavy lifting of altering your water pH, and the blend of both means no disturbance to your N/K ratios throughout grow and bloom – and no residue on the surface of your tank from using phosphoric alone in hard water areas. Then, with its own special characteristics, organic citric acid takes over to buffer pH changes over time, keeping your nutrient solution more stable, and helping handle calcium excesses in hard water areas.