pH Scale

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 & 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 - 8.3.

pH Fluctuations

pH Levels In Reef Aquariums

In reef aquariums containing corals, hobbyists maintain great success at a pH range of 7.8 to 8.5 with an alkalinity between 120 and 200 ppm. Within this range, we know it is best to keep things as stable as possible but some fluctuation is going to happen.

We also know that lower pH levels, below 8.3, suppress the rate of calcification within corals. This means that while you can safely house coral with a pH as low as 7.8 and as high as 8.5, you really want to shoot for a pH of 8.3, or as close as you can get, to maximize growth among our corals.

pH Fluctuations Are Normal

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 throughout the day. This is a normal occurrence in an aquarium and happens because of the rise and fall of dissolved carbon dioxide in the water. Reef-building corals and all types of algae take in CO2 to power photosynthesis. As the light level in the aquarium increases, the rate of photosynthesis rises, and more CO2 is removed from the water.

Less carbon dioxide - CO2 = less carbonic acid=increased pH

When the lights dim and photosynthesis stops, the CO2 level rises again, lowering the pH slightly. This daily cycle is harmless as long as the pH stays within the recommended range.

The ultimate goal for optimal coral growth is to minimize the gap in that fluctuation as much as you possibly can. If your pH never quite reaches 8.3 during the day and is dropping way down to 7.8 (or less) at nighttime, there are a few things to consider that will help raise the overall pH in your reef tank, reducing that gap and improving calcification among your coral. 

Alkalinity Additives
  1. 1 kg Reef Foundation B (Alk)
    Red Sea
    1 kg Reef Foundation B (Alk)
    $31.99 $31.99

  2. Reef Foundation B (Alk)
    Red Sea
    Reef Foundation B (Alk)
    From: $19.99

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 of the carbonates and lower alkalinity. If it is out of range, use an alkalinity additive to bring the alkalinity level up. It may take several doses and several days to raise alkalinity and stabilize the pH. If the alkalinity is within ideal range and your pH does not stabilize where you want it, something else is keeping the pH low.

Excess Carbon Dioxide

With energy-efficient insulation in our homes, scientists are discovering indoor CO2 levels are climbing to unhealthy levels because our homes are effectively sealed off from air exchange. In these situations, your protein skimmer is probably pumping CO2-rich air into your tank and helping to lower pH. The continuous injection of CO2-laden air like this could be the cause of your low pH. Alternatively, the CO2 build-up in your tank could simply be caused by inadequate aeration of the aquarium.

Here is a cool test to find out whether you just have too much carbon dioxide trapped inside your home or if you simply need to better aerate your tank water. 

Air test

A. Outside Air Test

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

B. Inside Air Test

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

Alternatively, 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. In either case, too much carbon dioxide in the aquarium water is your problem, but the exact cause will dictate a a different solution.  


If your aquarium simply needs more aeration, try adding an airstone to the filter sump or internal filter. You can also just increase the surface agitation in the aquarium (via a powerhead) to help promote proper gas exchange in the water.

If the air inside your home is CO2-laden, opening a window can be a simple solution to raising pH in the tank. At a minimum, if tank pH goes up after opening the window you can at least verify your previous test results, confirming there is too much CO2 in the inside air. That said, keeping a window open year-round is not always a viable solution, especially for those living in areas with temperature extremes.  

CO2 Scrubber Installation

Reducing Carbon Dioxide with a CO2 Scrubber

A CO2 scrubber is recommended when opening a window isn't viable and increasing aeration just won't cut it. We stock a variety of CO2 scrubbers including the TLF Phosban 150 conversion, BRS CO2 Scrubbers, and the Ice Cap CO2 scrubbers.

The exact installation may vary slightly but all of them work similarly to the Phosban 150 diagram above.  Air is drawn into the skimmer through the CO2 Scrubber where it comes in contact with CO2 absorption media. CO2 is effectively "scrubbed" from the air before it is aerated into your aquarium water via the skimmer. 

The Two Little Fishies CDX Adaptor is quite handy for DIY converting any media reactor with a 1/2" connection into a CO2 Scrubber.

Learn More About CO2 Scrubbers With BRStv

Reducing CO2 with a Refugium or Algae Scrubber

A natural way of removing excess CO2 is through photosynthesis which occurs while growing algae. There are a variety of ways to safely grow macroalgae and microalgae in your aquarium that provides great additional benefits, beyond just CO2 removal.  


Refugiums typically house macroalgae. As the macro grows, it will absorb CO2 along with nitrates and phosphates.  This means you will not only will it help to raise pH, but it will also remove nutrients. Refugiums will also provide a safe haven for beneficial microorganisms like copepods to thrive and reproduce, supporting the natural food chain in your aquarium. 

MacroAlgae Reactors

MacroAlage Reactors, like the Pax-Bellum and Tunze Macroalgae Reactor, are simply enclosed refugiums. They house macroalgae that grow and reduce CO2 levels while also absorbing nutrients. The only difference is the macroalgae is housed in a sealed reactor or container, making it easier to install in situations where space does not allow for a large refugium.  

Algae Scrubbers

Algae Scrubbers are a little different but give you the same benefits of CO2 and nutrient removal.  In this case, turf algae (a type of microalgae) is grown, which again uses photosynthesis to reduce CO2 and absorb nutrients. 

Refugiums, Algae Scrubbers, & Reactors
  1. ATS-300 Algae Turf Scrubber
    Adaptive Reef
    ATS-300 Algae Turf Scrubber
    $899.99 $899.99

  2. HOB-R Hang-On-Back Refugium
    HOB-R Hang-On-Back Refugium
    $142.99 $142.99

  3. Advanced Hang On Back Refugium PRO Series - Large
    Fiji Cube
    Advanced Hang On Back Refugium PRO Series - Large
    $249.99 $249.99

  4. Advanced Hang On Back Refugium PRO Series - Medium
    Fiji Cube
    Advanced Hang On Back Refugium PRO Series - Medium
    $229.99 $229.99

  5. Algae Scrubber - Small (20 Watt)
    Algae Scrubber - Small (20 Watt)
    $247.49 $247.49

  6. A.R.I.D. N18 Macroalgae Reactor (40-120 Gal. Sys.)
    Pax Bellum
    A.R.I.D. N18 Macroalgae Reactor (40-120 Gal. Sys.)
    $635.00 $635.00

  7. Small AquaFuge2 Hang on Back Refugium with LED Lighting
    CPR Aquatics
    Small AquaFuge2 Hang on Back Refugium with LED Lighting
    $266.99 $266.99