Injecting CO2

[JSW]

Hi,  If I am injecting CO2 and the pH is not dropping by about 1 point, is the injected CO2 "going to waste?"  I know I can increase the flow rate, but I am just wondering if it is necessary to see a pH drop for the CO2 to be effective.  Thanks in advance.

[Pepere]

If you are not seeing any drop in ph, your co2 may be rising straight to the surface and escaping… or your flow rate is just vastly inadequate.  In either case, it isnt doing a whole lot of good for the plants.  
 

Now an argument can easily be made for a co2 level below 30 ppm and

 consequently a ph drop less than 1.  
 

iirc the math correctly a ph drop of 1/2 is roughly around 15 ppm which is a whole lot better than no injection…

[Cory]

In my experience, good plant growth and ph dropping from co2, is overrated. It can totally be a tool. But I also see the average person have success with much less than 28ppm of co2.

[StephenP2003]

When the lights are on, dissolved CO2 in a planted tank never goes to waste. Our plants are desperate for it.  That said, any amount of added or produced CO2 (that dissolves into aqueous CO2 and doesn't just bubble out of the tank) will affect pH, and it's your pH and KH levels that are used to approximate dissolved CO2 levels. That 1-point drop is a super broad rule of thumb and is considered to provide a safe, non fish-killing level (but in reality there are far more variables that determine safe levels).  It's not about how much your pH changes, though; you determine CO2 levels based on your pH after CO2 has been going in the aquarium for a few hours, in reference to your KH (which doesn't change from CO2 injection due to the way carbon species interact in water, but that's another 3000 words of explanation).

There's a formula to determine CO2 levels based on pH and KH, as well as some caveats that can throw off the accuracy of that formula, but for the moment, I'll just show you the pH KH CO2 relationship chart. I'm happy to deep dive into the eye-glazing details and exceptions upon request lol.  But the chart is below. There are multiple versions of the chart floating around on the internet, but this one is the most comprehensive, lifted from the Barr Report (but I know that Tom won't mind). 

Ignore the color coding, especially the "Ideal" range. You really need to be observing your fish to know what's too much for them. 

So for example, I have a KH of 7 in my tap water, and in some of my CO2-injected tanks I inject enough to bring my pH to 6.8, which is around 33 ppm of dissolved CO2 (and technically in that "red" zone, but my particular fish are fine).

If you quantified that in terms of a pH drop (aka, worrying about your starting pH), it's not a very helpful frame of reference. My tap water comes out at a pH of 8.2, very CO2-poor. Once that water has spent some time in a seasoned tank with atmospheric gas exchange and all that respiration and nitrification and decomposition going on from the tank inhabitants, the pH stabilizes around 7.7 to 7.9 depending on the time of day and stocking levels (more living things = more respiration = more natural CO2 production). In any of these cases, bringing my pH down to 6.8 yields approximately the same amount of dissolved CO2. If we're framing it in terms of a pH drop, it's a drop of 1.4 from the tap, or less than a 1 point drop in a heavily stocked tank. And some of my CO2-injected tanks still maintain pretty high levels of CO2 at night when the solenoid is closed and the lights are off, and the pH might only rise to about 7.1 or 7.2 before the CO2 gets going again in the morning and goes back down to 6.8.  So that's a "drop" of 0.4, and a very very deceptive number if taken out of context.  

But as Cory said, you can do great things with far less dissolved CO2. If you're not trying to grow difficult plants that NEED high levels of CO2 injection, any amount of additional bioavailable carbon you can provide is going to contribute to faster growth, better coloration, just overall healthier plants.

In this tank, for example, I'm only maintaining about 15 ppm of CO2:

The MOST important factor in successful CO2-injected tanks is consistency. Plants have a much slower response and adaptation to changes in conditions compared to algae, and I've found the best way to farm algae is to crank up your light and CO2 levels for a while, then suddenly stop the CO2 without changing your lighting. Reliable CO2 uptake leads to more efficient photosynthesis --> leads to more efficient fertilzer consumption --> leads to more robust and healthy plants --> leads to consistent growth -->leads to poorer conditions for algae.  

[Pepere]

One also has to measure ph drop right…

your tank will hold quite a bit of co2 overnight until next on period.  You are not so much looking for a ph drop of 1 from right before the solenoid turns on until equilibrium is reached, but a drop of 1 from when equilibrium is reached and a fully degassed sample. 
 

ie take a sample of water and put it in a lidded container ans shake vigorously and then take off lid and let air out, and repeat a few times.  Best to do this outdoors as co2 levels outdoors is much lower than in most houses…

Dennis Wong site below has loads of good info on co2 levels and testing methods.

https://www.2hraquarist.com/blogs/choosing-co2-why/co2-fine-tuning-3-techniques

 

and 15 ppm does wonderful things for tanks.  30 ppm is getting up to the upper level of safe areas with a good buffer…. For most plants that level is not so much needed…

[StephenP2003]

On 10/22/2024 at 3:22 PM, Pepere said:

your tank will hold quite a bit of co2 overnight until next on period.  

Great point. I edited my post to mention that about one of my tanks where the pH only rises about 0.3 to 0.4 at night. I still say measuring the drop is pointless because of all the variables mentioned. It's all about the target pH. 

[Pepere]

On 10/22/2024 at 4:28 PM, StephenP2003 said:

I still say measuring the drop is pointless because of all the variables mentioned.

It along with the other cheap, and easy methods such as a drop checker, ph kh charts are all rather imprecise…,  but a method they are…

 

my original response was not so much about absolute values of co2 in the tank so much as saying that if ph doesnt budge at all between equilibrium and fully degassed, you might not be getting much co2 dissolved into the water…

a dirt diffuser putting out larger bubbles, no current to keep those bubbles in suspension long enough to get dissolved into the tank water, insufficient injection rate….  You should see “some” difference in ph between equilibrium and degassed…

Another point to keep in mind is that your PH test kit needs to measure a large enough range.  My low KH degassed water measures ph 6.6.  API only measures down to 6.0, so not too useful for measuring a ph drop of 1.