Low pH is a common water quality issue with reef aquariums. Fortunately, it’s an easy thing to fix once you understand the factors that affect pH in your tank. Many aquarists rush into making dramatic changes, sending their reef on a pH roller coaster ride. Don’t do it! Here’s what you need to know on how to raise the pH in your marine tank.

What is pH?

We’re not going to overwhelm you with chemical equations. But it’s good to understand a few “pH basics.” Think of pH as the level of acidity in your tank’s water. It’s the measure of hydrogen ions (H+) in the water. The lower the pH, the greater the level of hydrogen ions. You’ve seen the pH scale used with pH test kits. Unlike other parameters, the pH scale is “logarithmic”. What’s that mean? A pH of 7 is 10-times more acidic than water with a pH of 8. But pH 7 water is 100-times more acidic than water at pH 9. A relatively small shift in pH equals a large change in hydrogen ion (acidity) concentration.

Alkalinity and CO2 Regulate pH

In your aquarium, the pH is determined by the alkalinity and carbon dioxide (CO2) levels in the water. Carbon dioxide forms carbonic acid in water. Alkalinity is the bicarbonate and carbonate concentration in the water. Seawater contains about 125 ppm alkalinity. Marine salt alkalinity levels can range from 125 to 180 ppm, or even higher. Under ideal conditions the balance of alkalinity and dissolved carbon dioxide bring the pH to an average of 8.1.

pH Levels for Reef Aquariums

Fortunately for reef-keepers you don’t have worry about keeping the pH “locked in” at a particular level. Reef aquariums containing SPS and LPS corals are being kept with great success at a pH range of 7.8 to 8.5 with an alkalinity between 120 and 200 ppm.

pH Fluctuations are Normal

Graph of pH fluctuations rising during the day and lowering after dusk

If you’re using a pH probe to log your tank’s water quality, you may find the pH is lower in the morning and increases during the day. This is due to the rise and fall of carbon dioxide in the water. Reef-building corals and all types of algae take in carbon dioxide to power photosynthesis. As the light level in the aquarium increases, more carbon dioxide is removed from the water. This causes the pH to rise. That’s because carbon dioxide from the atmosphere can’t diffuse into the water fast enough to replenish what’s used by the corals and algae. When the lights dim and photosynthesis stops, the CO2 level rises, lowering the pH slightly. This daily cycle is harmless as long as the pH stays within the recommended range.

Alkalinity and Low pH

If your pH is too low, the first thing to check is the alkalinity level. Coral calcification will use up some the carbonates and lower alkalinity. If it is out of range, use a pH buffer to bring the alkalinity level up. It may take several doses and several days to raise and stabilize the pH. If this does not work, adding more buffer won’t help. Something else is keeping the pH low.

Excess Carbon Dioxide

You’ve probably seen the commercials for foam insulation and new energy-efficient windows. Everybody is looking for energy savings. But there’s a hidden downside to a tightly-sealed home. Scientists are discovering indoor CO2 levels are climbing to unhealthy levels. This means your protein skimmer is probably pumping CO2-rich air into your tank and helping to lower pH. The continuous injection of carbon dioxide could be the cause of low pH.  But the CO2 could simply be a build-up caused by inadequate aeration of the aquarium. Here’s a cool test to find out what the cause really is.

A. Outside Air Test

  1. Take a water sample and immediately test the pH
  2. Using an air stone, bubble air from outside your home into the sample for about 12 hours
  3. Test the pH (It should rise)

B. Inside Air Test

  1. Take a water sample and immediately test the pH
  2. Using an air stone, bubble air from inside your home into the sample for about 12 hours
  3. Test the pH

If bubbling inside and outside air causes a rise in pH, it means your tank just needs more aeration to drive off excess CO2 to raise pH. There’s nothing unusual about the carbon dioxide level in your room. However, if the outside air test increases the pH but the inside air test doesn’t, you know there’s too much CO2 in the room.

Increase of aeration to raise pH

If your aquarium needs more aeration, try adding an airstone to the filter sump or internal filter or increase the surface agitation in the aquarium to help promote proper gas exchange in the water. Otherwise, the solution to excess indoor CO2 may be as simple as cracking open a window.

Reducing Carbon Dioxide with a CO2 Scrubber

Installation diagram of a CO2 scrubber to raise pH
Two Little Fishies: PhosBan Reactor 150, CDX System Adaptor, and CDX Carbon Dioxide Adsorption Media

If simple steps aren’t practical, a CO2 scrubber is recommended. Make your own scrubber with a Phosban reactor or similar media contactor. Fill the reactor with a CO2 absorption media, like Two Little Fishies CDX media. Use a CDX adaptor to connect the protein skimmer airline to the reactor. As the skimmer draws air through the scrubber, CO2 is removed, which allows the pH to rise.

Reducing CO2 with a Refugium or Algae Scrubber

A natural way of removing excess CO2 is through photosynthesis. A refugium with macroalgae is a great way to lower carbon dioxide and provide a source of live foods like copepods. If you like the high-tech approach, a stand-alone algae scrubber like the Pax Bellum macroalgae reactor is worth a look. The unit is a self-contained system for growing Chaetomorpha. The algae remove nutrients like phosphate while stripping out excess CO2.

Final thoughts

No matter how you choose to raise pH, do take your time. Allow the tank to respond to what you’ve done. Impatience will lead you to try one thing after another, usually with defeat and frustration. Find out the root cause of the low pH and take the appropriate steps. In time your tank’s pH will increase and stabilize at a healthy level.

Having trouble with High PH? Check out our artcle on How to Lower pH In Your Aquarium.