Is the pH in your reef aquarium too high? Don’t panic or reach for a pH adjuster just yet. Before doing anything it’s important to understand the fundamentals of pH and the factors that affect it in your tank. Relax! This isn’t going to be a chemistry lesson and there’s no math involved. Here’s what you need to know about how to lower pH in your reef tank.

What is pH?

pH is simply the level of acidity in the water. Technically, pH is the concentration of hydrogen ions (H+). The lower the pH, the greater the concentration of hydrogen ions. The pH scale is quite familiar to anyone with a pH test kit. It’s also “logarithmic”. Here’s an example. Water with a pH of 7 is 10-times more acidic than water with a pH of 8. That same pH 7 water is 100-times more acidic than water at pH 9. As you can see, a small shift in pH can mean a big change in hydrogen concentration. In saltwater, the pH level is determined by the alkalinity/carbon dioxide (CO2) balance.

Recommended pH level for Reef Aquariums

There is no defined “perfect” pH level for reef tanks. Successful reef aquariums loaded with thriving corals are typically kept at a pH range of 7.8 to about 8.5 and alkalinity between 120 and 200 ppm. If the alkalinity or CO2 level are incorrect, the pH will drift out of the recommended range.

Daily pH fluctuations

pH Fluctuations

Algae and corals use carbon dioxide to drive photosynthesis. Carbon dioxide diffuses into the water from the atmosphere and is released into the water from marine life (respiration) and the decomposition of organic matter. Carbon dioxide forms carbonic acid which pushes the pH down.  As the light level in the tank increases, photosynthesis uses up carbon dioxide, allowing the pH to rise. This daily cycle is normal and harmless if the pH stays in the proper range. If you test the pH in the early morning before the lights are on then again toward late afternoon, you’ll probably see this pH fluctuation.

Other Causes of High pH

Calcium additives can also raise pH. Kalkwasser and some two-part calcium additives can quickly push the pH to high levels. The rise is only temporary because the water chemistry will self-adjust by absorbing CO2 from the air. You may have to cut back on the dosing if you see a dramatic or long-lasting rise in pH after dosing.

Using Aeration to Lower pH


If your alkalinity level is within the recommended range and the pH is too high, the CO2 level is probably too low. Adding aeration to increase the CO2 level in the tank usually re-balances the pH to a normal range. Think of it this way: the CO2 level in the aquarium is lower than the CO2 concentration in your home’s air. It’s not diffusing into the water fast enough to bring the pH down to a normal level. Aeration adds the room’s CO2 to the aquarium water. Try an air stone in the sump or increase the water’s surface agitation to increase CO2 diffusion into the water to help lower pH in the aquarium.

Adding Carbon Dioxide

Some aquarists have had success lowering pH by adding CO2 in the form of carbonated “seltzer” water. Unflavored seltzer is bubbly because it’s infused with CO2 gas. Adding about 5 ml per gallon lowers the pH by approximately .2-.3 pH units. Add it to the sump or in an area of high water flow.

Adding Acids

Avoid using strong acids like muriatic or sulfuric acids. These are commonly found in freshwater “pH Down” products. The acid will lower pH and use up alkalinity too, so a better choice is white distilled vinegar. It will lower pH directly and again as the acetic acid is consumed by bacteria. The biological process produces CO2 and replaces the lost alkalinity. Add 1 ml per gallon in an area of water flow but don’t overdose. Adding too much can over-stimulate the bacteria resulting in an excess of CO2, low pH, low oxygen and even cloudy water from a bacteria bloom.

Final Thoughts

Results will vary per aquarium due to many factors that may contribute to CO2 levels. These are just a few suggestions on how to lower pH in the aquarium. If you have any other tips please feel free to share.