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Whereas a bubble counter is quite suitable for controlling the amount of CO2 added to a planted tank, a permanent test is important for measuring the actual amount of CO2 dissolved in the aquarium water. There are many optically quite pleasing variants that meet the aesthetic standards of an aquascape.
Usually, these permanent tests are made of glass. They are filled with an indicator fluid. The color of this indicator shows the amount of CO2 dissolved in the water. Yellow signifies that there is too much CO2, a blue hue indicates too low a level. An intense green is a sign for the correct amount. There are two different indicator fluids at your disposal. The CO2 Check by Aqua Rebell is available for a standard amount of 20 or 30 mg/l of CO2. The liquid indicator turns green when the respective value is reached. Always adjust the amount of carbon dioxide to the requirements of your tank — it depends strongly on the plant species and the technical equipment (mostly the lighting system) of your aquarium. For planted tanks with moderate light and a corresponding stock of plants, the 20 mg/l CO2 test makes a lot of sense. A higher content of 30 mg/l of CO2 is recommendable for strongly lit tanks with a large number of fast-growing stem plants.
Determining the CO2 content with the help of so-called CO2 tables (in dependance of the pH and the carbonate hardness of the water) does no really make much sense, as they are rather theoretical. In real life, there are too many influencing factors that may have an impact on the ratio of CO2, the pH and the carbonate hardness.
The indicator liquids by some other manufacturers need to be mixed with aquarium water prior to use. Those are only of limited usefulness for a truly exact determination of the CO2 content as well. They are less precise and depend on the carbonate hardness of the water you use. The Aqua Rebell CO2 Check testing liquids have a fixed carbonate hardness and thus always show the correct color change — independent of your tap water. There's no need to pre-mix them, they come ready for use.
If you are unsure whether the test shows the right shade of green you can always revert to a reference test. Permanent tests like the Double Check v2 by Cal Aqua Labs have two chambers, one with the value that is actually measured and one with a referencing color. In this case you need to use the indicator as well as the referencing solution by Cal Aqua Labs to get correct results, as the colors of other tests may vary.
Maintenance: No matter which test you use, renew the indicator fluid in the drop checker after 4 to 6 weeks to make sure that it always shows the correct value.