This permanently indicating test uses colouration to show the aquarist the pH value and the CO2 content of the aquarium water at any time. Thus, you can read off 24 hours a day whether the pH value is within the optimal range and whether the plants have sufficient CO2 as their main nutrient available during the day. 

How exactly does the test work? 
The CO2 dissolved in the aquarium water diffuses in the container of the permanent test, passes into the air column containing the indicator liquid and then dissolves in the liquid. 
This creates a pH value, which results in the discolouration of the indicator liquid. The colour displayed corresponds to a certain pH-value and depending on the carbonate hardness of the water you can read off whether the resulting CO2 content is too high, too low, or optimal. 
If another pH-lowering or increasing substance (e.g. pH-Minus) apart from CO2 is added to the aquarium water, it cannot diffuse through the air column in the permanent test and thus does NOT affect the indicator liquid. The pH value then displayed would therefore not be correct. It only shows correct values, when there are NO other pH-influencing substances in the water besides CO2. 

How long does it take until the test displays correctly? 
Due to its design the test is somewhat sluggish, because the CO2 gas needs to first "migrate" through the air column in the test vessel, then into the indicator liquid and can only then be displayed. It can therefore take up to one hour until the test shows the current CO2 content of the aquarium water for the first time. 

Recommended CO2 content: 
freshwater aquarium (community aquarium): 15-35 mg/l; Lake Malawi-Tanganyika aquarium: 4-10 mg/l; 
plant aquarium with few fish (aquascaping): 20-35 mg/l