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The 2hrAquarist legend Dennis Wong
tested the Hanna Test Kit vs a $3,000+ CO2 testing device and found accurate results in low kH aquasoil based tanks when compared. $30 for CO2 readings vs $3,000+!
It's a simple kit that comes with simple instructions, two differently sized vials for different detection ranges, a titration syringe phenolphthalein indicator, and an NaOH solution of some unknown concentration to titrate with.
For the range of detection on the
Hanna CO2 test kit we want (0-50ppm):
- Add 10mL of aquarium water to the small flask. Add 1 drop phenolphthalein and swirl gently.
- With the titration syringe (markings are "reversed" from traditional syringes), add the NaOH solution dropwise and swirl gently until color goes from clear to pink. Don't shake/agitate, or you will de-gas your CO2 concentration.
- For the 50ppm detection range, multiply the number on the syringe by 50 to get your result (if you used 0.4mL NaOH to turn the vial pink, you would have 20ppm CO2).
My findings:
It seems to be "accurate" based on my comparisons with 1) drop checkers, 2) my pH reading/meter, and 3) the inhabitant-response (fish gasping at 35+ppm, shrimp uncomfortable at 30ppm+). It's not
perfectly accurate (±5ppm), but it does a good "instant" job at giving some information in about 1-2 minutes. The kit also comes with enough for MANY, many tests (at least a hundred). For $30 I think it should become standard equipment for most CO2-injecting hobbyists.
I was worried about interactions with other acids in the flask, and I'm sure there are a few, but in clean, low-kH water, it doesn't seem like a problem. I found some stuff online from published journal articles about how there are no other worrisome strong acids to compete with the NaOH and that the CO2 readings from NaOH titration are generally accepted as accurate (in peer-reviewed journals). Not a silver bullet for CO2 testing, but really helps me get an understanding of my tank and keep it dialed in.
How I use it
- If I want a quick glance of CO2 concentration for whatever reason in that moment (e.g., my fish are lethargic or shimp are at the surface).
OR, the more complex way using it to dial in CO:
- Day 1 of CO2 on a new tank: I keep an eye on inhabitants and their behaviors (to make sure I don't over-inject) and I test 2-3 times throughout the day. The most important is the test done right before the CO2 shuts off, which would be the maximum CO2 level for the day. As long as I didn't get to "true yellow" on my Drop Checker, I hopefully plateaued in terms of injection, finding the equilibrium between injection and off-gassing.
- On Day 2: CO2 comes on 1-3 hours before the photoperiod. I compare the CO2 readings at the very start of the photoperiod, once during, and once at the end. A quick estimative plot on the graph should tell me if I'm at/around equilibrium.
- For example, if my tank tests 6-10ppm CO2 before the solenoid turns on, then 20ppm when the lights come on, then 25ppm halfway through the photoperiod, and 30ppm by the end of the photoperiod, I know I'm somewhere in the proper "zone" (highlighted in pale yellow):
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- On Day 3: I double-check the measurement at the start of the photoperiod, during, and at the end of the CO2 injection. As long as all 3 are close/approaching 30ppm, we're good!
On general timing of CO2 injection in this hobby:
I continue to hear/read from reputable sources about
how plants use the most CO2-per-hour in the first 3-4 hours of the photoperiod, which is why it's so important to have CO2 injection reach/near 30ppm @ equilibrium between injection and off-gassing.
Many beginners inject too little, or start injecting too late relative to their photoperiod. As Wong
states in his article here,
I think many users fall in the "Low injection rate" category, where they are only hitting optimum CO2 levels in the afternoon (when plants are using less of it to begin with!)
:
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But they don't realize that you can fix that
either by 1) increasing surface agitation (for increased off-gassing) as well as increasing injection rate:
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OR by 2) Starting your CO2 injection way earlier than you think. For example:
Many hobbyists read "turn CO2 on 1 hour before lights" and end up with this:
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Where they are only reaching optimum levels waaaay too late in their photoperiod, when they really should either start the CO2 way earlier, or start their photoperiod way later, like this:
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To bring it all home (what I think):
- The Hanna CO2 test kit is a lot cheaper than a reliable pH pen/meter/computer, and requires no calibration fluids or calibration itself.
- It's been tested to be somewhat accurate when compared to a very expensive aquatic CO2 testing device.
- You can use something like the Hanna CO2 test kit to test for whether you're starting CO2 early enough, or whether you are even reaching equilibrium between off-gassing and injecting CO2.
- Like all titration, it can be prone to user error. If the goal is light pink, everyone will see light pink differently. Just do your best to be consistent with your readings!
- It costs $33+ shipping and will likely last the average user a few years (if the reagents don't go bad before then). Worth it!
Just another tool in the toolbox to help us with our tanks. Let me know what you think!
