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CO2 Drop Checker - Effective measurement of CO2

by Andy Kindon | Posted in How-to Guides | No comments yet. | 694 Views on this post

What is a CO2 Drop Checker?

Ensuring correct level of CO2 within a planted aquarium is often considered one of the most difficult things to get right – unfortunately it also happens to be one of the most critical aspects of maintaining a nature aquarium. Too much CO2 and you risk asphyxiating livestock; too little and the plants begin to suffer, giving algae an opportunity to take hold. Fortunately, a CO2 drop checker can be used to give an approximation of the levels of CO2 dissolved within the aquarium water – with the ideal level being around 30ppm (parts per million).

Drop Checker










A drop checker, in its simplest form, is nothing more than a pH test kit housed either in a plastic or glass vessel within the aquarium itself, with an air pocket between the tank solution and the drop checker solution. pH test kits are often used by aquarium hobbyists to check the acidity of aquarium water – with the solution turning from blue (alkaline), through green (neutral) and red (acidic). In much the same way, when combined with a drop checker, a pH test kit allows us to estimate the level of CO2 in the aquarium water, as the pH of the water changes dependent upon the ppm of CO2 within the water.

pH Scale

As CO2 evaporates from the aquarium water and into the drop checker, the pH of the drop checker solution becomes more acidic, turning the solution yellow. It’s important to note however that CO2 isn’t the only source of acidity within aquarium water - waste from fish, fertilisers and even the hardness of the water used to fill the aquarium all have an effect on the acidity. It is therefore unadvisable to fill a drop checker with tank water, as the pH measured isn’t always a direct result of the CO2 injected, resulting in inconsistent and inaccurate drop checker readings.

A solution to this is to fill the drop checker with distilled water that has been adjusted to a specific kH value.  As such, the change in pH changes can be attributed directly to CO2 that evaporates from the tank water and comes into contact with the drop checker solution, rather than the factors mentioned previously. It is common practice now for drop checkers to be filled a mixture of Bromoythmol Blue (the pH test solution, also known as reagent) and 4dkH solution. With this solution, tank water containing 30ppm of CO2 causes the drop checker solution pH to drop to 6.6 – resulting in a green to lime green colour. The colour of the solution can then be used to gauge the level of CO2 within the aquarium water – yellow being too much, lime green being optimal, and blue being too little.


Setup and maintenance

You will need


  1. Fill the drop checker 2/3 full with Flo 4DKH+ Solution.
  2. If using Flo 4DKH solution, you must then add 3 drops of reagent to the drop checker, and shake the drop checker gently to ensure the solution is mixed. If the colour isn’t easily visible, add more drops.
  3. Invert the drop checker so it is oriented in the upright position. Be careful as some solution may leak out.
  4. Mount the drop checker in the aquarium by pushing it under the water in an upright position. This will create an air gap between the tank water and the solution held in the drop checker.
  5. Change the solution weekly to fortnightly to ensure accuracy.


  • Mounting the drop checker in the vertical centre of the aquarium should give an indication of the ‘average’ level of CO2 in the water. However, moving the drop checker to other areas of the tank can be helpful when trying to monitor CO2 levels throughout the tank.
  • It can take several hours for the drop checker solution to accurately portray the level of CO2 within the water. It is therefore advisable to assume that the colour indicated by the drop checker shows the level of CO2 from approximately two-to-three hours ago - not the current level. As a result, changes to the CO2 injection rate should be made infrequently and in small increments.


Getting the perfect shade of green.

One of the shortcomings of CO2 drop checkers is that they are slow at reacting to CO2 level changes. It is therefore imperative that you use a drop checker with patience, and allow sufficient time for the colour of the solution to change, before adjusting the level of CO2 injection.

As previously mentioned, the optimal level of CO2 for planted aquariums is 30ppm – resulting in a lime green drop checker solution. As too much CO2 can be fatal for livestock, it is much safer to inject too little than too much. It will take several days of insufficient CO2 to cause any detrimental effects to the plants, but just a few minutes of too much CO2 can kill everything in your aquarium - especially sensitive creatures like shrimps. As such, starting with a low level of CO2 injection and gradually working up is considered the safest approach.

It is hard to advise a starting injection rate of CO2, as it is very much dependent on tank size and the efficiency of the diffuser being used. However, the following rates can be used cautiously as a starting point.

  • Below 100 Litres - 1 bubble per 5 seconds
  • 100 to 200 Litres – 1 bubble per 2 seconds
  • Above 200 Litres – 1 bubble per second

Note: Always keep a close eye on livestock during the initial 24 hours of setup – any signs of breathlessness or gasping at the surface should be taken extremely seriously, and the CO2 should be immediately turned off and additional aeration provided.

Once a base injection rate has been decided, the drop checker should be left until the CO2 concentration reaches its equilibrium level – where the CO2 concentration stays constant because the injection rate covers any uptake of CO2 by the plants, and any evaporation of CO2 from the water. You will know this has happened when the colour of the drop checker solution doesn’t change for a period of 4 consecutive hours. The process could take up to 8 hours, and should be done during the day while the lights are on.

Now that a base injection level has been set to work from, a note of the drop checker colour should be made. If the solution is too blue, make a small increase to the bubble rate and leave for another 8 hours before you check again. Similarly, if the solution is too yellow make a small reduction to the bubble rate and leave for another 8 hours. Repeat this process until the drop checker remains a lime green colour permanently.


But wait… green doesn’t mean everything is perfect.

Whilst the drop checker may be a lovely shade of lime green, the plants could still be suffering from lack of CO2 as CO2 levels in the tank are not necessarily even in all locations. For example the lowest corners of the tank may receive much less water flow, and thus less CO2. It is therefore optimal for high planted aquariums to have a high flow rate and efficient water distribution. The recommended filtration flow rate for a planted tank is roughly 3x the size of the tank per hour. However, as filters rarely provide their quoted turnover you should work on the assumption that your filter delivers around half of its quoted performance. With this assumption in mind, you should aim for a filter than provides roughly 6x the flow rate of your tank e.g. for a 100 litre tank, a filter that provides a quoted turnover of 600L/hr is recommended.


Going advanced.

Once you have achieved stable CO2 injection using the method above, a way to increase the length of time that your CO2 canister lasts, is to turn the CO2 off at night. This method is trickier to perfect than the 24-hour method, but benefits include reduced CO2 usage and risk of asphyxiation to livestock. It also requires extra hardware in the form of a regulator solenoid valve and timer.

Plants only need optimal CO2 levels during the day, when they are at peak growth. When the lights are switched off, the plants stop using most of the CO2 in the aquarium water. As such, it is perfectly safe to turn the CO2 off a few hours before the lights, as the CO2 already in the water will be adequate for the plant’s needs. Overnight the level of CO2 in the water will drop, turning the drop checker to a green-blue or blue colour. Turning the lights on and initiating plant growth before the drop checker is back to green could induce algae. To combat this, turn the CO2 back on a few hours before lights on. This method will ensure that whenever the plants require it, the CO2 level is optimal.

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