The wrong way to read the pH/KH chart.

Problems with the pH/KH/CO2 table

The pH-KH-CO2 table shows that as the CO2 level in the aquarium increases, the pH will decrease. However, the table assumes that the only factors affecting pH are CO2 and alkalinity. In reality, most tanks contain a variety of other substances that affect pH in addition to the basic KH.

Many other factors contribute to the pH of the tank: fertilisers, tannins and organic by-products. Matching a single point value of pH and KH to obtain the CO2 level assumes that KH and CO2 are the only factors determining pH. This is not the case - in almost all tanks there are other elements that cause the pH to be lower than what the expected KH would extrapolate to. Many weak acids will lower the pH without significantly changing the KH. For example, the popular use of Aquasoil in aquariums immediately renders the table useless, as the fulvic and humic acids will lower the pH significantly.

So while the table is useful as a reference of how the pH/KH relationship works. In reality, no hobbyist will have enough accurate chemistry data on what is in their water column to use it in any meaningful way.Measuring real world data with an expensive CO2 analyser and comparing the results with the pH/KH table produced few usable examples.

Worse still, hobbyists have no way of knowing whether the chart will work for their tank or not because they have no way of taking into account the various factors that can affect the pH in their tank.

Average CO2 levels in non-CO2 injected tanks

A cup of plain water sitting on the table will measure 0.6ppm of CO2. While atmospheric CO2 measures around 400ppm, after applying Henry's law for gaseous diffusion, the amount of free dissolved CO2 in standing water is around just 0.6ppm.

However, aquariums often have other contributors to CO2, such as respiration from bacteria/animals /plants. Decomposition from thick aquasoil beds or large filter systems with lots of decomposing organic matter can increase CO2 levels in the aquarium. On average, however, non CO2-injected tanks will measure around 2-3ppm of free dissolved CO2.

Using an OxyGuard CO2 analyser gives us a reading of 5ppm CO2 in this crowded aquarium. The OxyGuard CO2 analyser provides highly accurate readings of free dissolved CO2, the probe measures the carbon dioxide content of the water directly by detecting the carbon dioxide partial pressure in the water - its readings are independent of pH measurement.

This planted tank similarly has a CO2 level of around 5ppm, probably due to decomposition occurring in the aquasoil base.

In the majority of aquariums we have measured that do not use CO2 injection, those without a soil base measure around 2-3ppm CO2. A few tanks measure up to 6ppm CO2, but this does not represent the average case.

Right approach in using pH as a gauge for CO2 levels

As a very general guide, you should aim for a relative pH drop of 1 point from the time when CO2 injection has not yet been switched on to the time when it has been switched on and the CO2 has risen to a high, stable equilibrium point. This will put the CO2 level in the tank at around 30ppm. If you are careful, many tanks can aim for a drop of 1.2 pH instead. This method is simple but accurate if your pH tests are well calibrated. For example, if your tank starts with a pH of 7.2 without CO2 injection - you should aim for a pH of 6.2 during peak CO2 saturation. There are complications associated with his method described in this in-depth article on how to measure CO2 levels.

Always be present to observe your animals when adjusting CO2. Fish will show signs of lethargy if CO2 rises to uncomfortable levels and at critical levels they will gasp for air at the surface. Certain species will be more sensitive to CO2 levels (e.g. discus) than others. Therefore, the CO2 target must be relative, based on observation of the tank inhabitants, and not just an absolute value. When releasing animals into the tank, always release them during the period when the CO2 injection is switched off. If levels build up gradually during the next CO2 cycle, they will have some time to adapt.

It takes about a week for plants to fully re-programme and optimise their enzymes to match the CO2 levels available. The true measure of what is a good CO2 level must be made by directly observing the plants over a couple of weeks. Stunted leaf tips, thin stems and new leaves that are smaller in size compared to older growth are all strong signs that CO2 levels are low (if light and nutrient levels are good).

Measuring CO2 levels by observing plants can be difficult for newer aquarists because it requires knowledge of what is a good growth form for a particular plant. Comparing pictures with online examples is a start, but you need to build up observational experience over time to become really good at detecting whether CO2 levels are good.

Head here for a more in-depth article on measuring CO2 levels in the aquarium.