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Injecting Enough?
Above: Consistent and sufficient CO2 levels are central to vibrant, durable Dutch-style 2Hr tanks.
If you have invested in CO2 injection, a good question to ask is: am I injecting enough? If your drop checker is always green or the bubble count is above, say, 3bps, does that mean you have enough CO2?
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.
Perform these 5 steps:
- Start CO2 injection 2hrs before lights come on.
- Just before lights come on, take a sample of tank water and measure its pH.
- Take the same test-tube above and shake vigorously for 3 minutes to de-gas dissolved CO2.
- Measure the pH of the de-gassed water and compare it with the initial pH (Step 2).
- Target a 1pH difference between the 2 readings.
Note:
- Step 2 would be the peak saturation / low pH point.
- A 0.7 to 0.8 pH drop is sufficient for most plants / tanks. A full 1pH drop puts you in the "Grow Anything" zone. But it is quite difficult to reach.
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 (see note 1 below).
With a 1pH drop, this puts CO2 levels in the 35ppm (parts per million) zone. For most tanks, this is the "Grow Anything" spot that is both sufficient and safe for livestock (see note 2 below).
For faster growing farm tanks like the one above, we target closer to 45-50ppm of CO2.
This method uses the known relationship between pH and KH. Importantly, we are interested in the relative pH drop, not the absolute number, which will vary from tank to tank.
So what does ‘green’ in my drop checker say?
It tells you that you have some CO2. But because the reading is highly dependent on the exact placement in the tank (and the flow), it doesn't tell you if you have enough, or more importantly - how much. Since CO2 is the #1 "nutrient" and component of plant mass, it makes a world of difference whether you are providing 10ppm or 20ppm (twice that amount) of CO2. Precision matters.
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.
3 different bubble counters, 3 different bubble counts. In the above setup, we took videos of 3 randomly selected bubble counters and attached them to the same CO2 regulator for 30 seconds. We repeated the experiment by combining the bubble counters in a daisy chain. It is easy to see that the answer to "is 3bps enough" is simply "maybe".
Is there another way?
Professional CO2 measuring devices aside, the Relative pH Drop Method may feel complicated, but is surprisingly consistent. You can read about this method here. Setting up and maintaining a drop checker also takes effort, with less reliable results. Accurately counting bubbles in a bubble counter also takes some work and is much less informative. Since CO2 plays a large role in plant health and growth, it is often worth the extra effort!
In general, we find that 80% of aquarists using CO2 injection are under supplying CO2. It's easy to look at the bubble count and think "oh, that's a lot!"...but in reality, the majority tend to inject too little, not too much :)
Note 2: The pH-KH relationship is non-linear. If your tank water measures 0-1dkH, you need to aim for a relative drop of about 1.5pH to be within reach of the 25ppm CO2 target;
Note 1: While pH flux from CO2 exchange does not generally harm livestock, it can bring the resultant pH outside the preferred range of individual species. Always check first, and be present to observe signs of stress. Good gaseous exchange allows you to raise CO2 levels safely without endangering livestock. Read more here.
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