pH Swings in an Aquarium

One of the best features about our aquarium controller is the ability to see graphically the swing in our pH levels. Daily swings are expected, but we are also seeing much more drastic shifts and have had to research and do some investigation to discover the cause.

There is a daily swing in pH simply from the natural photosynthetic and respiration cycles of the tank. During the day the photosynthetic process converts CO2 and water to carbohydrate and oxygen. The tank then has less CO2, and a higher pH. The respiration process, done by all organisms, converts the carbohydrates back into energy and produces more CO2 and thus a lower pH. The respiration process is being done constantly, but is more noticeable at night when the lights are off and photosynthesis is not affecting the pH. This is where the daily swing comes from.

One way to help equilibrate pH swing is to keep an adequately high KH level in the tank. KH (Carbonate Hardness) is also known as alkalinity, but not the pH measurement “alkaline”. KH measures the concentration of hydrogen carbonate, as well as carbonate ions in water. The concentration of these ions gives a good idea of the capability of water to maintain proper pH levels, because the carbonate hardness acts as a buffer against fluctuating pH levels. We have been testing and dosing regularly for this.

Another way is aeration, which helps to drive off CO2 if there is too much, or absorb it if needed. This gas exchange happens mainly on the water surface, so one of the best way to aerate is to place powerheads that circulate the water from the bottom of the tank to the top with a relative amount of force. From my reading however, this only helps a little and does not prevent these pH swings completely.

The typical daily pH swing for a normal daily cycle ranges from 0.1 to more than 0.5 pH units. What we have found in the short amount of time in using the pH probe and viewing the graphs is that we have the problem of low pH, rather than elevated pH. Correcting elevated pH can be done with increased aeration, addition of CO2 directly via soda water, or adding white vinegar and limewater to the aquarium. Because our pH has been running on the low side, we have been dosing with KH to help balance the swing, and have adjusted our powerheads a little to help with circulation, and hopefully aeration.

Another adjustment we have made has been to add the air line to the skimmer that we had not been previously utilizing. We ran an air hose from the skimmer through a networking jack, and out through the same hole that the heating/cooling unit comes into the house from outside. We have noticed improvement with this addition.


We believe our main reason for low pH levels comes from using our unventilated gas stove in our fairly new home. The amount of oxygen from outside able to come into the home if we have our house closed up (all doors and windows) is very little as the house is so new and so well insulated. When we use the oven or stove, the flame absorbs oxygen in the house, which also affects the aquarium. Now, this seems to be quite silly, trying to correlate using an everyday household item to pH swings in the aquarium. However,  the graphs of our aquarium controller show that when the stove is being used, the pH drops. Coincidence? When we noticed this happening we began opening the windows and doors to the house, as well as the hood and cabinet doors on the aquarium after the stove was used and could see the pH rise via the pH graph, as oxygen was coming into the house, and thus into the aquarium.

Sure enough, researching this subject brought up articles about elevated CO2 levels in newer homes with un-vented appliances. The CO2 levels in your home can be up to twice the amount as that of outside!

Now that we are aware of this issue that not only affects our aquarium, but also ourselves and our pets, venting our stove is something we will be considering for the near future. As for now, on most nights, assuming the weather is cooperative, we have been opening the windows to our home to help increase oxygen levels.

Since adding the air line to the skimmer, opening windows, dosing KH, and trying to increase aeration, we are seeing improvement in our pH, and pH swings. Once winter comes and we use our furnace and stove more, we may have to do more for our aquarium’s oxygen levels, but for now, these have been our findings and solutions.



This entry was posted in Informational and tagged , , , , . Bookmark the permalink.

3 Responses to pH Swings in an Aquarium

  1. Doris Moon says:

    Oh Gosh, my cook stove isn’t vented however; after I was poisioned by Co2 I leave all windows opem 1/4 “, now I am thinking clearly again..well, I think so ..

  2. Collin says:

    The most important thing regarding pH in aquariums is to keep the levels consistent. You will do way more damage to your tropical fish and water parameters when you try to raise and lower your pH. Keeping your fish tank pH consistent ensures that your fish have a steady environment.
    Really nice post! Thanks for sharing this to us !

    • Amanda says:

      Thanks for your response Collin! Keeping a reef tank can certainly be a challenge. I think that is why we love it! Thanks for replying! Have a Merry Christmas!

Leave a Reply

Your email address will not be published.