Injecting Enough?

The answer is: maybe.

If you keep simple plants (and are relaxed about plant color and shape), this may be enough. If you are trying to keep HC Cuba, exotic Eriocaulons, or are trying to achieve top coloration and form, there is a good chance that your CO2 may not be enough.

This is because both drop checkers and bubble counters vary widely in design and give different signals in different tank conditions. What may be sufficient in one situation may not be sufficient in another.

So when users ask, "Is my CO2 enough? and refer to their bubble count or drop checker, we are often faced with the challenge of politely pointing out that "hmmm...that may not be the case..."

So how do I gauge that my CO2 is sufficient?

For all 2Hr Tanks, the method we use and recommend is the Relative pH drop Method. As CO2 is dissolved into the water column, the pH of the water will drop as Carbonic acid is produced due to dissolved Carbon dioxide. This relationship is linear, so we can use the change in pH to chart how much CO2 is dissolved. A 1pH drop correlates to 30ppm of CO2 in the water column. This relationship holds true for KH ranges of 1 - 10 dKH. 

1. Start CO2 injection 2hrs before lights come on.
2. Wait a few hours for CO2 levels to stabilize and hit peak saturation.
3. You will know that the tank is at this state when the pH of the aquarium stops changing when CO2 is on.  This is the 1st pH reading.
4. Take a water sample from the aquarium and stir/shake vigorously for a couple of minutes to degas it. It takes a few minutes in a well ventilated area to degas a sample fully. 
5. Take the pH reading of the now degassed water sample. This is your second reading.
6. Your target is to have a 1 pH rise in the degassed water sample. This translates into 30ppm of dissolved CO2.
7. If your pH rise is more than 1 pH, it means you are injecting more than 30ppm of CO2. If your pH rise is less than 1pH, it means you are injected less than 30ppm of CO2.

While it has the hassle of a science experiment, it is the simplest, most reliable method that can be consistently performed at home without expensive professional equipment. It does require a good pH test kit and a prior KH water test to see if your tank water measures between 1-10 dKH.

30ppm of CO2 is a very safe zone for livestock. Most common fish can tolerate CO2 levels well into the 50ppm range, so targeting 30ppm is a conservative zone that works well across a large range of aquariums.

A few demanding species such as Centrolepis drummondiana and Eriocaulon quinguangulare, Hygrophila sp Chai prefer higher levels of CO2. In aquariums growing these species we target a 1.2 to 1.4 pH drop to hit above 40ppm of CO2.

So what does ‘green’ in my drop checker say?

It takes surprisingly little CO2 to turn the drop checker green. For standard 4 dKH solutions, it turns a deep shade of green from as little as 10+ppm of CO2. While lower CO2 levels can grow easier species of plants, the plant selection of modern day aquascaping has greatly expanded, and many aquatic plant species do better in more optimal CO2 levels. Color interpretation of drop checkers also change depending on light conditions and surrounding colors of the aquariums.

Color interpretation by the human eye can be very imprecise.

What about bubble count?

Bubble counting is also an unreliable method of measuring CO2 sufficiency, not only because different bubble counters have different bubble rates, but also because the actual CO2 level is highly dependent on the effectiveness of diffusion and flow, not just the injection rate. You can have a high injection rate, but if you have a clogged or inefficient diffuser, you may not be adding much CO2 at all.