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Cal Aqua CO2 Double Drop Checker Reviewed

The Cal Aqua Double Drop Checker is very effective and if handled with reasonable care, will last for many years.

Cal Aqua Double Drop Checker by Cal Aqua.

Those keeping highly technical (‘High-tech’) planted aquariums realize the need for accurately measuring the carbon dioxide (CO2) in their tank’s water. High-tech aquariums normally include the application of aquatic plant specific substrate, carefully selected lighting and supplemental nutrients including CO2. The high-tech approach allows hobbyists to create strikingly beautiful underwater gardens that in many cases, would not be possible.

These aquariums are true works of art. Some hobbyists depend solely upon bubble counters to visually observe the amount of CO2 being introduced into the water. The downside to only using a bubble counter is that it leaves the hobbyist guessing how much CO2 is being injected into the tank. In the past, some hobbyists would adjust the amount of CO2 being introduced based upon whether or not the fish were gasping at the surface.

Excessive CO2 levels will kill fish. Hopefully, that irresponsible and inhumane method is no longer used. One method of visually (and fairly accurately) monitoring the CO2 level is through the use of an inexpensive drop checker.  The drop checker is installed inside the aquarium and left there.  Drop checkers are glass or clear plastic vessels that are filled with an indicator solution and trapped air.

The indicator solution consists of KH standard (4-degree) water and Bromothymol sulfone phthalein (Bromothymol Blue) which is a pH indicator for reactions between weak acids and bases. A drop checker works by the CO2 in the water seeking equilibrium. As the CO2 gas is absorbed into the indicator solution, the pH becomes lowered which changes the solutions color. The color changes are not rapid and take a period of hours to show.

The color of the solution in the drop checker can be compared to a color chart to determine if more or less CO2 is needed in the aquarium. A double drop checker is actually more accurate because it allows an in-tank, side-by-side comparison of the indicator solution against an adjacent reference solution within the vessel. The in-tank, side-by-side comparison eliminates the subjectiveness of color comparison charts which can be affected by aquarium and room lighting.

The Cal Aqua Double Drop Checker sold by Green Leaf Aquariums solves the color comparison problem.  Here are my observations after using these double drop checkers for many years:

  1. The top cavity holds the indicator solution.  As CO2 is introduced into the tank, it becomes absorbed by the indicator resulting in a color change.  The top cavities solution is compared to a reference solution in the bottom cavity.  The reference solution is calibrated to 30 parts per million (PPM) of CO2 which is considered to be a good level for planted tanks.
  2. For convenience, Green Leaf Aquariums sells ready-made indicators and reference solutions.  Some hobbyists choose to make or obtain their own which is fine as long as care is taken to ensure the indicator and reference solutions are accurate.
  3. Green Leaf Aquariums suggests changing the solutions if they become clear or no longer show color changes.  I change mine about every three months which is probably excessive.  Use care when handling the glass drop checker.  Although it is durable, glass will break if handled roughly.
  4. Filling the drop checker vessel takes practice.  Getting the correct amount of reference solution in the bottom cavity is simple but the upper cavity is challenging.  I found taking a syringe with a very small hose attached allows precise filling without the need for tilting or shaking the vessel.
  5. The suction cup included with the drop checker is very high quality and will not wear out in a few months.

The  Cal Aqua Double Drop Checker is very effective and if handled with reasonable care, will last for many years.  Enjoy your planted aquarium!

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Article Categories:
Fish · Health and Care