Resource Hanna CO2 Test Kit: My experience so far (and why you're probably not starting CO2 injection early enough)!

[Naturescapes_Rocco]

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):
    
    
    
• 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,

"This scenario can give rise to a situation where CO2 levels are very high by day's end, with fish gasping at the surface. Hobbyists are puzzled as to how fish can be gasping and yet their plants are not getting enough CO2. This is unfortunately an extremely common scenario in the planted tank world when people use low injection rates coupled with poor gaseous exchange."

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!):

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:

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:

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:

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!

 

[HarryL]

Is it just me having trouble accessing the attachment 7500? Says "The requested page could not be found."

 

[Naturescapes_Rocco]

Is it just me having trouble accessing the attachment 7500? Says "The requested page could not be found."

Try reloading, I should have fixed it!

The graphic is my image here:


It shows that even though I inject CO2 starting at 5am, I don't have the lights come on until 10am so the water is fully saturated with CO2 at 30ppm at equilibrium between injection and off-gassing. I turn CO2 off about 2 hours before the lights go off, since the plants are using less CO2 at that point anyway, and it takes a while for the off-gassing to occur to any meaningful amount.

 

[HarryL]

So did you intentionally have CO2 on for 5 hrs at lower bps because you have lower surface agitation? Or what is the reason you don’t opt for it to reach 30ppm 1-2 hrs before lights are on?

 

[Naturescapes_Rocco]

Great question, shows you know what this is all about!

(I sort of answer it in this thread: Testing the Aqua Rocks Colorado Acrylic Horizontal CO2 Reactor on a large tank)

Basically, the problem with horizontal reactors is that the injection rate is tied to the injection maximum amount of CO2.

If I run my reactor on max power (50% full of CO2), my injection rate is really fast (1.5 hours to reach ~30ppm) but it continues to inject, and the maximum CO2 injeciton/offgassing equilibrium is close to 50ppm (too much for my fish). The two options I have are to either 1) reduce the max power of my reactor (which also reduces the injection rate), or 2) agitate the surface more to "flatten" the curve through off-gassing, like this (line A):


I can't really agitate the surface much more than I already am -- I already have two filters with outlets at the surface, creating ripples across the entire surface of the tank. I suppose I could add a cooling fan to hit the surface, or add a wavemaker pointed at the surface, but then I might have too much current for my liking.

The simple option for me was to run my reactor at only ~1/3rd full, and just start my injection way earlier in the day! I still get the curve-flattening from off gassing eventually (line B in the above graph), but I don't need to worry about gassing out my fish because the reactor really caps right at the proper 30ppm CO2.

So, I end up with this:

And all is well!

I really wish this was better explained in the hobby. Credit to Dennis Wong for making it more widespread and approachable.

 

[HarryL]

I did a majority of my learning from Dennis’ website as well, so many useful information, I pretty much read every article he has.

I never really thought about measuring the concentration after the lights are on. I use a diffuser and when I first started injecting, I measure the pH just like how Dennis suggested. If I get a 0.8-1.0 pH drop from a completely off-gassed sample, then I let it stay that way without any further adjustments. I actually forgot to run the light on schedule once, it stayed off while CO2 remained on for the whole day, all shrimp and fish seem perfectly fine when I came home. I wonder if the small amount of light from the window enabled enough photosynthesis to help offset the CO2 level, or the CO2 just happened to be plateauing with whatever off-gassing rate the tank has.

With one of my tanks I currently do different light intensity during different time of the day to control algae. Since the plants use the most CO2 in the first few hours, do you think it makes the most sense to start with higher intensity as well, then lower the intensity?

 

[Naturescapes_Rocco]

Since the plants use the most CO2 in the first few hours, do you think it makes the most sense to start with higher intensity as well, then lower the intensity?

I think this is a great topic worth experimenting with! In relation to CO2 use, it makes sense, however, what if plants uptake CO2, but actually perform the majority of their photosynthesis in the afternoon? Is that why we see more oxygen bubbles in the midday/evening, even though plants uptake more CO2 in the first part of the photoperiod?

It would be fun to set up two identical tanks, and try this very idea. Who knows, maybe it is actually more beneficial to have stronger light in the second half of the photoperiod for some reason! If anyone else knows I'd love to hear.

 

[Burr740]

Great thread

 

[Koan]

The Hanna CO2 test kit is a lot cheaper than a reliable pH pen/meter/computer

And word is getting out! Hanna has it on back order they don't expect any more to ship for at least 3 weeks

 

[Naturescapes_Rocco]

Here's the Hanna CO2 test kit instructions (two pages)!

One thing I've been thinking about: my readings may be super accurate because I use a reactor, so the CO2 is 100% dissolved into my water. Could this kit be as accurate for tanks using a diffuser? Most likely, but might be something to note.