Drop Checker in Planted Aquarium

Many hobbyists asked the same question over and over again about how to measure CO2 level in their planted aquarium. On most occasions, some hobbyists suggest using ph/kH table. It is not a wrong suggestion but unfortunately the table only work if 100% sure there is nothing in the aquarium that can alter pH or kH beside CO2. This is impossible in planted aquarium since we use wood or soil that will reduce pH. In addition, we have sand and rock in our aquarium that tends to increase kH. This is where drop checker come in handy.

pH/kH table

Drop Checker Design
Drop Checker come in many different designs. Below are the typical designs that can be found in the market.

Understanding How Drop Checker Works

The drop checker is actually pH tester but instead of measuring the pH of the aquarium water, it measures the pH of 4dkH reference solution called it indicator solution. Refer to picture below.

When CO2 is injected into the aquarium, at certain point the concentration of CO2 in the aquarium water is higher than the CO2 concentration in the drop checker. In this condition the CO2 will gas out from the aquarium water into the air pocket of the drop checker. As CO2 enters the air pocket, it will be absorbed by indicator solution. The same goes when the CO2 concentrations in the aquarium lower than CO2 concentration in the drop checker. CO2 from indicator solution will gas out and absorb by the aquarium water. This is because the gas was seeking equilibrium point between indicator solution and aquarium water.

As the CO2 absorb by the water, it will lower the water pH and this is the basic principle of drop checker. Using pH indicator solution (Bromothymol Blue) and kH reference solution (4dkH) user will be able to find out roughly the CO2 concentration in the aquarium without worry about contaminant that will change pH or kH.

The indicator will change color as follow:
- Yellow: high CO2 level (>35ppm)
- Green: proper CO2 level, this is what we aim for (25ppm - 35ppm)
- Blue: low CO2 level (<25ppm)

Drop Checker Misconception
Most if not all drop checker manual will tell you to use aquarium water. Try not to do that. Its the same reason as why not using pH/kH table, you do not know what all the contaminant that alter the water pH or kH.

For example, if you are using ADA Aquasoil, the water pH will be around 6.2 and water kH will be 0dkH. If you use this water, drop checker will always show yellow even though you never inject any CO2.

You can buy 4dkH reference solution from ebay or you can make some yourself. What you need is distilled water and sodium bicarbonate (baking soda). If you have kH tester you can mix distilled water and sodium bicarbonate until you get 4dkH. Alternatively follow the following formula: dilute 6 gram of sodium bicarbonate in 1 liter of distilled water. It will give you 200dkH. Take 10ml of 40dkH solution and dilute it with 490ml of distilled water to get 500ml of 4dkH reference solution.

Some forum post remind me that some drop checker brand such as Cal Aqua Labs or Dennerle does not require to dilute it with kH = 4 solution

Drop Checker Placement
Drop checker placement can be quite tricky. First of all, it must be put under the water roughly around 10-15cm from the water surface. Be sure to put drop checker away from the flow path of CO2 enriched water. In other word do not put your drop checker near CO2 diffuser. Also do not put drop checker in front of the filter outlet if reactor or inline diffuser is being used.

Drop Checker Limitation
The drawbacks of drop checker are
1. It is not real time measurement device. It takes approximately 1 to 2 hours before you can observe the color change. Thus you might not be able to catch sudden surge of CO2 need during photosynthesis peak period.
2. It wont tell you the exact ppm, although in our intent and purpose, it is not necessary to know the exact ppm
3. Does not work well if the CO2 introduced using misting method because CO2 bubble will go into the drop checker and cause false reading.

Nevertheless drop checker is one of the most accurate and affordable CO2 measurement device available for planted aquarium.