Does CO2 Affect PH? 

A newbie in the planted tank hobby would be bombarded with terms like pH, gH, kH etc. While this may sound a little too technical, understanding what these represent would make it seem much easier. To simplify things, these are just names we call for certain measurements of certain elements in the water. These are necessary to ensure that the tank has the most favorable conditions for plant and fish growth. 

By now hobbyists would have been already introduced to the noon of using CO2 supplementation/injection in the planted tank. There is actually no other way to make the plants become healthier. This makes most hobbyists feel uneasy as carbon dioxide is a poisonous gas in large amounts. Luckily that concern has been addressed with proper methods of injecting CO2 and with products that help check the levels of this gas in the tank, like bubble counters, regulators and drop checkers. Another big concern is if the CO2 can actually affect the water’s pH. PH is important to fish on so many levels. 
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In this article, we are going to answer the question if carbon dioxide change the pH of water and whether this is dangerous for the tank.

The answer is yes!

Yes, carbon dioxide and pH are related to each other. What happens when co2 dissolves in water? CO2 when dissolved in water will lower the water’s pH. Depending on how much CO2 is dissolved in water, pH going down or going up means a lot to a planted tank or any tank for that matter. It is not some harmless phenomenon that normally occurs. An experienced aquarist may already know that pH is important to fish. A swing in pH by just a few degrees in the opposite direction may spell trouble for the tank. Let us first understand what pH is and how it can affect the tank’s overall health. 

What is pH?

PH (potential of hydrogen) is the way the acidity or alkalinity is measured. It goes from 0 to 14. The neutral is 7. Anything lower than 7 is considered acidic while anything above 7 is alkaline. Water, pure water with zero dissolved solids, is always neutral. If the water would now have dissolved solids (measured in TDS) the pH could swing left or right depending on what is dissolved in it. Calcium and magnesium are alkaline and, if dissolved in water, can pull the pH up. Acids have the opposite effect. So when we say that pH changes with CO2, it is another way of saying CO2 turns the water acidic. Oxygen to a certain degree has the opposite effect of CO2 when it comes to the water’s acidity or alkalinity. Oxygen when dissolved in water actually makes the pH go up. 

test kit

As we already know why does CO2 decrease pH, let see how it affects the planted aquarium. CO2 affecting the acidity of water has both positive effects and detrimental ones. It would now be up to the aquarist how to go about injecting CO2 and not worry about its detrimental effects of it. So what are those effects? 

Positive Effects 

Aquatic plants would generally require so, slightly acidic water to thrive. In nature, these plants would generally be found in streams and lakes where there could be leaf lier, eventually turning into humic acid. Plants love these types of waters for as long as it does not get too acidic. By introducing CO2 in the tank not only would the aquarist provide enough of carbon dioxide for plants to photosynthesize, he would also inadvertently be creating the ideal water for plants to thrive in. 

aquascape by SidScapes

The problem is CO2 injection, the method most commonly used is only turned on when the lights are on because this is the only time when plants do photosynthesis and would actually require the gas. If the lights are off, plants would not require carbon dioxide. The common method would be turning CO2 on and off with the lights. There would then be an airstone and pressurized air that would run when the lights are off to introduce Oxygen into the tank. Even if there is no airstone, Oxygen will get introduced into the tank through the gas exchange if there is surface agitation. CO2 makes water pH go down. Oxygen makes water pH go up. So, the tank has a constant, daily swing in pH. Plants can tolerate this as long as the fluctuation is not that great. This is bad news for algae though. Algae, commonly encountered in planted tanks, cannot tolerate acidic water. Moreover, they would find it hard to thrive in waters with fluctuating pH. 

Negative Effects 

Not all fish can tolerate fluctuating pH. It may not kill them right away but it is sure to have a detrimental effect later on. This can be resolved by selecting the fish to be put in the tank. Hardier varieties should be selected. Refrain from fish that are too sensitive. Do not put fish that are normally found in high pH in a planted tank. For sure the tank will see this fish degrade. The fact is, not all fish are suitable for aquascaping. Some fish require harder water and higher pH. Some fish require colder temperatures and some prefer warmer temperatures than what is normally found in an aquascaped tank. Choose the fish wisely. 

Affect on the hardscape

Another aspect that needs to be considered would be how low pH affects hardscape, especially rocks. It has been found that acid or low pH levels in water can help accelerate the release of heavy metals from rocks. This may not sound much as CO2 does not influence that great of a pH swing. It does turn the water slightly acidic and with metals being released from rocks, this seemingly harmless scenario could turn potentially dangerous for the flora and fauna once the heavy metals accumulate to intolerable levels. 

hardscape by Jordan Stirrat

Influence of pH swings on the livestock

Some experts tend to think that a pH swing in the tank is unimportant and does not really affect the fish. This is based on the readings they have taken on their own tanks which are then compared with the readings with other expert tanks as well. Mostly these are readings from a tank the primary focus is on the fish which means a tank filled with oxygen-rich water and with little CO2. A planted tank is different. Surface agitation is limited so as not to let the CO2 escape into the atmosphere. So the tank may be rich in Oxygen but it is also rich in CO2 as well. That in itself is already a trigger for stress. One would never know how certain fish would react if another stressor (pH swing) is introduced. 

A general swing in pH is stressful to fish. Some fish as explained earlier may be able to tolerate it. Most of the healthier ones do. Some fish though when put under a drastic swing will experience pH shock. Bear in mind that fish are found in waters with stable pH. These waters do not have their pH swing drastically and if ever there was a swing it would not affect every part of that body of water. Natural bodies of water are just too big and the activities in it just too vast to try and compare it to an enclosed environment like a planted tank. To be on the safe side, refrain or minimize or try to solve issues that would bring stress to fish. Stress in animals is just like stress in humans. It can create all sorts of problems health wise.

Constant CO2 dosing

A more stable way of applying CO2 is to apply it below the normal levels and apply it so it is present in the water 24/7. This makes for a much more stable pH in the tank. CO2 is not enough to influence a huge swing, and the water stays slightly acidic as CO2 is constantly being pumped to it 24/7. It would provide just enough CO2 for plants to thrive. The catch is not all plants will thrive in this type of setup. Lights would need to be toned down to control how much CO2 the plants would need. Higher light levels would mean plants would need more CO2. The easier-to-grow plants are the ones that are usually used for this. 

What is a drastic pH swing?

A pH swing is generally not a cause for concern in an aquarium, especially if it is not a huge swing. It happens all the time. A stable pH does not mean pH will remain at a constant reading. pH stability in aquariums and as far as fish are concerned can include minor swings in pH. Minor in this case is an increase from 0 to .03 or from zero to .05. An increase in pH by one full unit is already a 100% increase. A 7 pH increase to an eight flat is already a very drastic change. 


There are a lot of theories and methods based on these theories but the fact is, they will remain theories as no proper studies have been done on the subject. They have not been validated. All the data we have as a hobby are mostly based on observations of what is going on in the planted tank and no two planted tanks are exactly alike. Low pH and high CO2? Or opposite? It is now up to the reader and the hobbyist on which theory to follow and treat as law.

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