pH swings, low pH and CO2 in planted tank

CO2 injection must be handled carefully in any planted aquarium. Elevated levels can stress and eventually kill fish. Symptoms of stress include reduced activity levels or hiding, more severe symptoms include persistent surface or high flow areas, with the most severe symptoms manifesting as fish swimming just below the waterline with a slowed response to stimuli. CO2 levels take time to build up - so it is always preventable to spend time observing the tank when adjusting CO2 levels. However, many people underestimate the time it takes for CO2 levels to build up - in some setups it can take up to 4 or 5 hours before CO2 saturation reaches peak levels.

CO2 tolerance also varies between fish and shrimp species. For example, discus are more sensitive to high CO2 levels than tetras. Discus begin to show signs of distress when CO2 levels approach 40 ppm, while Cardinal tetras do not show any signs of distress even at levels above 60 ppm. For shrimp, Caridina shrimp are more affected than Neocaridina shrimp - very high CO2 levels can cause spawning to slow down.

However, CO2 injection is very easy and safe to handle if done well. This is especially so for tanks that are setup to have good gaseous exchange from the start (read the separate post on this). Having good gaseous exchange gives a lot of leeway for injecting CO2 vigorously.

Are pH swings from CO2 harmful to animals?

There is widespread concern that the pH changes caused by CO2 injection could harm livestock. This issue is actually more complicated than it looks. It may or may not be an issue in your tank.

The main reason why pH stability is often cited as important in aquariums is that pH levels normally reflect alkalinity (KH levels / carbonate hardness). Alkalinity affects the osmoregulation of fish / livestock. Sensitive shrimp, for example, are affected by sudden changes in carbonate salt concentrations in the water and dislike KH fluctuations of more than 3 degrees in a short period of time.

A change in KH will always reflect as a change in pH. However, the reverse is not true, pH can change significantly without a substantial change in KH, such as when pH changes due to CO2 injection. The KH in CO2 injected tanks remains consistent even as the pH falls and rises with the changes in CO2 levels. As long as the alkalinity (KH) is consistent, the animals will be alright.

Difference between pH and KH

It is the KH flux, not pH, that has a major impact on fish and animals. However, in most tanks it is easier to get a pH reading than to test for KH, and the pH reading has become the proxy for people to estimate whether their KH is stable or not.

However, if the pH is changed without a change in KH, as in the case of CO2 injection, the fluctuations in pH will generally not affect the animals. Weak acids, such as those produced by dissolved CO2, do not move KH much, there is no change in dissolved salt concentrations in the water column and therefore no significant effect on livestock osmoregulation from this point of view.

Excessive CO2 levels and extreme pH levels (where acidity itself becomes an issue) will still affect livestock, but these are separate issues from pH fluctuations caused by CO2 per se.

Neocaridina shrimps breed and thrive in many CO2 injected planted tank setups where the daily pH swings are more than 1 degree.

Demonstrate of pH swings that have no impact on livestock

If we do large 60-80% water changes in our CO2 injected planted tanks, the pH changes a full 1.0 unit in 20-30 minutes or less, yet we never lose sensitive shrimp or fish if the other parameters are kept consistent.

Fluctuations in pH ( 1 full point + ) are also very common in nature, as CO2 levels build up overnight due to decomposition, but are quickly reduced during light hours.

Fish from acid peat bogs will do well in a tank where the pH drops from 7 to 5.8 during CO2 injection, as the low point of the cycle (pH 5.8) is within the range of their natural living conditions. However, alkaline water fish may not tolerate the dip into the pH 5.8 range well. It is not the swing, but the pH itself that is outside the natural range of the fish. The same alkaline water fish can survive a comparable pH swing from 8.5 to 7. It is important to distinguish the effects of a pH change from the effects of an extreme pH value in itself.

Click here to learn more on how the different water parameters affect fish.

Does the low pH levels 'crash' the bacteria cycle in the tank' ?

The short answer is no. Bacterial colonies adapt to such levels. Thousands of low KH aquasoil tanks have low pH levels of 5+ to 6+. Yet they have no problems with either animals or plants. Read this article on how bacteria adapts to low pH environments.