CO₂ Curve Testing With Hanna + pH – 40–50 PPM

[TheBurn]

Got inspired by @Naturescapes_Rocco So I decided to do a full day of testing on my CO₂ levels yesterday for my UNS 60U before I add livestock, and I put together a graph. It was a pretty interesting test. My CO₂ comes on at 12 PM (3 hours before lights), lights run between 3–9 PM, and CO₂ shuts off at 8 PM. Injection during this test was 66 bubbles in 30 seconds, which equates to roughly (2.2 BPS)

I used an Apera PH60 for the pH curve and a Hanna HI3818 CO₂ titration kit to independently check CO₂ concentration

Current water parameters are GH 5 and KH 0

Hanna results:

12 PM (pre-injection): 8–10 ppm
3 PM (lights on): 41 ppm
8 PM: 49–51 ppm

pH went from 6.47 pre-injection → 5.61 by lights on, giving me a 0.86 pH drop by the start of the photoperiod. From there, it continued down into the ~5.5x range and plateaued through the rest of the photoperiod, putting the total drop at roughly 0.9–1.0 pH

Planned livestock is Cardinal Tetras, Pygmy cories, a few Amano shrimp, and eventually Caridina shrimp as the tank matures

What was interesting about this test was that as the photoperiod started, I was right where I wanted to be. It held a pretty steady curve, but later in the evening, just as the lights were about to go off, levels reached a little higher than I'd like. Ideally, I'd like to be somewhere around 40–45 ppm throughout the entire photoperiod, so I slightly adjusted my injection rate down to about 60 bubbles every 30 seconds, which equates to roughly (2 BPS)

I probably could've done just fine with livestock at my current rate, as I know guys like @Dennis Wong and Tom Barr run their tanks around ~50 ppm peak, but I know some shrimp, especially Caridina, can be sensitive to higher CO₂ levels. I'd rather be a little more conservative and play it safe. Plants are all doing very well

I'm going to do another full-day test at the new injection rate and see how the tank reacts. Hopefully I can balance the curve a little better and stay closer to that 40–45 ppm range throughout the entire photoperiod

I may also need to increase the CO₂ duration and start injection about 3½ hours before lights on instead of 3 hours. Ideally, I'd like CO₂ to already be around my target concentration when the lights come on rather than continuing to build afterward

Curious what those of you who run higher CO₂ with livestock think about this approach, especially those keeping shrimp. Has anyone else ever done a similar test?

 

[JayP]

It would be valuable to know what your fully degassed ph is so you know the true drop in ph and how it correlates to your co2 ppm levels. I imagine that 0.86 ph drop at lights-on is more likely over a 1.0 ph drop from fully degassed resulting in that 41ppm reading.

Also valuable to know is what level of surface agitation you have. A little higher agitation may result in your number stabilizing a bit lower than the 50ppm at end of day.

 

[Naturescapes_Rocco]

great post! Don't forget that at 0KH, the Hanna kit somewhat over-estimates your ppm CO2. When I had the Oxyguard CO2 meter, I found that at 0KH the Hanna kit was off by 5-10ppm CO2. At 1KH, it was closer to 0-5ppm, and at 2-5/6 KH, it was spot on accurate.

So my guess is you're injecting pretty perfectly! No harm in injecting another 30 or even 60 minutes earlier; it'll just mean you get to your true equilibrium faster.

 

[TheBurn]

It would be valuable to know what your fully degassed ph is so you know the true drop in ph and how it correlates to your co2 ppm levels. I imagine that 0.86 ph drop at lights-on is more likely over a 1.0 ph drop from fully degassed resulting in that 41ppm reading.

Also valuable to know is what level of surface agitation you have. A little higher agitation may result in your number stabilizing a bit lower than the 50ppm at end of day.

Good point. I actually did a fully degassed test previously. I aerated a tank sample until the pH stabilized, and the final degassed reading was 7.30. So using that as the baseline, the 5.61 pH at lights-on would actually represent about a 1.69 pH drop from fully degassed, which lines up better with the Hanna reading of 41 ppm.

As far as surface agitation, I'm running a UNS Delta 90 with a neo lily pipe positioned to give me slight, consistent surface agitation. Although I do wish the flow was a little stronger. I also have a UNS Mini surface skimmer that cycles on for 10 minutes, 3x a day.

I could definitely experiment with a little more surface agitation as well. My main goal is to get the curve as stable as possible around 40–45 ppm throughout the photoperiod rather than continuing to climb toward 50 ppm late in the day. I already backed the injection down slightly from 66 to 60 bubbles per 30 seconds, so I'm planning to map another full day and see how that changes the curve before making any other adjustments.

 

[snowballs99]

You could run an air stone on to stabilize co2 a bit more. Or a surface skimmer etc, they kinda do the same job.

 

[TheBurn]

great post! Don't forget that at 0KH, the Hanna kit somewhat over-estimates your ppm CO2. When I had the Oxyguard CO2 meter, I found that at 0KH the Hanna kit was off by 5-10ppm CO2. At 1KH, it was closer to 0-5ppm, and at 2-5/6 KH, it was spot on accurate.

So my guess is you're injecting pretty perfectly! No harm in injecting another 30 or even 60 minutes earlier; it'll just mean you get to your true equilibrium faster.

That's really interesting and actually has me wondering if I decreased my injection rate prematurely . I wasn't aware the Hanna kit could overestimate by that much at 0 KH, especially since my goal was around 40–45 ppm.

I only made a small adjustment from 66 bubbles/30 sec down to 60, so I think I'm going to leave it there for today and run the exact same full-day test again. That should give me a good comparison and show exactly how much that small reduction changes the curve. If it ends up being too conservative, I marked my original setting so I can easily go back to 66.

Really appreciate you mentioning the OxyGuard comparison. That's exactly the kind of real-world data I was hoping to get from posting this.