Lowering Alkalinity With Water Changes. If you’re trying to raise ph, soda ash is a the choice to use, under the guidance of the water treatment process. Alkalinity is water's capacity to resist acidic changes in ph, essentially alkalinity is water's ability to neutralize acid.
The corals in your tank will reduce the alkalinity over time as long as you do nothing. Water ph measures the amount of hydrogen (acid ions) in the water, whereas water alkalinity is a measure of the carbonate and bicarbonate levels in water. Total alkalinity strongly influences your ph level, so if your ph is off, be sure to check the alkalinity.