Measuring CO2 in your aquarium with a drop checker: tips & tricks

The key to a beautiful, healthy plant tank? That’s adequate CO2 in your aquarium. Unfortunately, this is also one of the hardest aspects to master. In my experience (and the many e-mails I get from readers) I would estimate that 90% of all algae problems is caused by CO2.

Fortunately, there are some methods that can help you estimate how much CO2 you have in your aquarium. One of these methods is the use of a  “drop checker“.

How does an aquarium drop checker work?

The drop checker is filled with a “drop checker indicator mix“, which changes colour based on the pH value of the indicator mix. The drop checker is then placed in the aquarium, creating an air bubble in the neck. This way you get a separation between the liquid in the drop checker and the aquarium water. The CO2 from your aquarium passes through the air bubble and gets into liquid the indicator mix, changing its pH value. This is the biggest advantage of the drop checker: there’s no contact with the aquarium water so no contamination by all kinds of by-products in the aquarium water that can affect the final measurement, for instance phosphates, acid substances from aquasoils,…

In other words: CO2 is the only factor that affects the pH of the drop checker indicator mix. If you add more CO2 then more CO2 passes over to the liquid inside the drop checker, making it more acidic and thus lowering its pH value. Suppose the pH value of your indicator mix is 6.6 pH by adding CO2 and the KH is 4, then you know that the CO2 value is about 30ppm according to the pH/KH table. The indicator mix will turn green, meaning that a green colour indicates 30ppm CO2.

Which color does your drop checker need to be?

Based on the color of your drop checker you can estimate the CO2 value of your aquarium water:

• Blue: not enough CO2
• Green: enough CO2, about 30ppm
• Yellow: too much CO2, can be toxic for fish and shrimp

Keep in mind that this is still an estimate. Color differences are not always obvious, and furthermore, you still need to take care of the balance between your CO2 and lighting you are using. Suppose you have 30ppm of CO2 but you use a ton of light above your tank then you may still have too little CO2 just because you are using such a large amount of light. The higher the amount of CO2, the stronger plant growth is but also the stronger the CO2 demand becomes. In this case, I would recommend reducing the light intensity.

Where to place your drop checker in your aquarium?

Somewhere at the bottom, about 2 to 3cm from the soil. Why? Because CO2 rises. If you place your CO2 drop checker at the top, chances are you will get a larger CO2 reading than what your plants are taking in. This is because there will be much less CO2 at the bottom of your tank than at the top. Often the drop checker at the top colors a nice green, but if you then place it at the bottom, it will be rather blue-green. Slightly too little.

I recommend placing CO2 drop checker somewhere at the bottom and then increase the CO2 slightly in case the color is a bit blue. When adjusting your CO2, always keep a very close eye on your fish! Do this once a week at most to avoid fluctuations in CO2.

Have you already seen my drop checker kit?

I sell a drop checker kit which contains everything to measure the CO2 in your aquarium:

1. A nice glass drop checker
2. A bottle of drop checker indicator mix

Here’s how the kit looks like:

Each kit will last about 33 weeks if you replace the indicator mix weekly (with each water change).