Aren’t these two statements about the JBL CO2/pH Permanent Test contradictory? The special indicator shows the CO2 level independently from substances in the water, such as humic acids or nitrate, which influence the pH value. The pH permanent test only works when no substances, such as pH-Minus or peat, which influence the pH values, are added.
No, the two statements do not contradict each other.
The indicator reacts to the pH value of the test liquid. The indicator in the JBL CO2/pH-Permanent test only changes colour because of the carbon dioxide which has diffused from the water into the test liquid.
Humic acids or other acids have a pH-reducing effect. As a result the real pH value in the aquarium water can be lower than that shown in the indicator, which is only influenced by the diffused carbon dioxide inside the water.