The wrong way to read the pH/KH chart.

Problems with the pH/KH/CO2 table

The pH-KH-CO2 chart demonstrates that as CO2 levels increase in the aquarium, pH falls. However, the underlying assumption of the table is that the only factors affecting pH are CO2 and alkalinity. In reality, most tanks have a large variety of other substances affecting the pH besides the base KH.

Many other factors contribute to the tank's pH value: fertilizers, tannins & organic by products. Matching a single point value of pH and KH to get the CO2 level assumes that KH & CO2 are the only factors that determine pH. However, this is not so - in almost all tanks, there are other elements that causes the pH to be lower than what the expected KH value would extrapolate to. Many weak acids lower pH without significantly changing the KH value. For example, the popular usage of aquasoil in the aquarium immediately renders the table useless, as the fulvic and humic acids lowers the pH significantly.

Thus, 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 utilize it in a meaningful way. Measuring real world data using an expensive CO2 analyzer and comparing results to the pH/KH table returned few usable examples.

The worse thing is that hobbyists have no way of knowing whether the table will work for their tank or not, because they have no way of accounting for the various factors that can influence 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/livestock/plants. Decomposition from thick aquasoil beds or large filter systems that have a lot of decomposing organic matter can raise the CO2 levels in the aquarium. Averagely though, non CO2 injected tank measure around 2-3ppm of free dissolved CO2.

Using an OxyGuard CO2 analyzer gives us a reading of 5ppm of CO2 in this crowded fish holding tank. The OxyGuard CO2 analyzer gives highly accurate free dissolved CO2 readings. The probe measures the carbon dioxide content of the water directly by detecting the carbon dioxide partial pressure in the water - its readings are not dependent on 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 non-CO2 injected aquariums we measured, the ones without a soil base measure around 2-3ppm of CO2. A few tanks measure up to 6ppm of 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, aim for a 1 point relative pH dropfrom the point when CO2 injection is not yet turned on to the time after it has been turned on and CO2 has risen to a high, stable equilibrium point. This will put the tank's CO2 levels 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. So 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 around to observe your livestock when you are tuning CO2. Fishes show signs of lethargy when CO2 rises to uncomfortable levels and at critical levels, they will be gasping at the water surface. Certain species will be more sensitive to CO2 levels (Discus for example) than others. That is why CO2 targeting must be relativedepending on observation of tank livestock and not just an absolute value. When releasing livestock into the tank, always release them during the period that CO2 injection is turned off.When 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-program and optimize their enzymes to match available CO2 levels. The true gauge of what a good CO2 level is must be done by direct observation of plants across a couple of weeks. Stunted leaf tips, thin stems and new leaves that are smaller in size compared to older growth are all severe signs that CO2 levels are low (if light & nutrients levels are good).

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

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