CO2 injection

[Jeff_F]

   I've been doing some reading about CO2 to learn more about how it impacts the planted aquarium.   There seems to be two different approaches to determining how much CO2 is in the tank.  The George Farmer book says to take Ph readings with and without co2 injection and you should expect a 1 Ph drop at 30 ppm.  So if the without (nighttime) Ph is 7.1 then you should see a Ph of 6.1 while injecting CO2.    Tom Barr discusses using the Ph/KH chart to measure the CO2 ppm.  So If my KH is 3 dkh then my Ph should be about 6.4 - 6.5 for 30 ppm CO2.  

  If I look at my tank with a KH of 3 dkh, my nighttime Ph is 7.1 so if I add CO2 during the day to bring the PH down to 6.1 the ph/kh chart says the CO2 ppm would be in the 70's.   I figure all the fish would be dead at that point!!  However, my ph does go down to about 6.1 during the day and everything seems fine.   So, what am I not understanding?  

Edited August 25, 2023 by Jeff_F

[nabokovfan87]

cc @Mmiller2001 help us out!

Yeah, CO2 can be a bit fickle and getting things mathematically sound is pretty frustrating.  It's something I deal with on my own.

A few key steps:
1.  Make sure the diffuser is working and not blowing out large bubbles
2.  Makee sure those bubbles are being blown around the aquarium via circulation and not being diffused directly to the surface.
3.  Keep an eye out for things like localized algae growth based on where the co2 is being diffused and try to avoid things like BBA as a result of co2 use.

So.... the PH thing.  The main point here is that CO2 will impact PH.  KH plays a role in understanding what PH value makes sense, but the big takeaway here is that you want to see a PH drop while you're diffusing CO2.    That drop correlates to the amount of impact the CO2 is having on the system.

The Barr chart is basically using KH (closely tied to PH values) and trying to mathematically measure the CO2 amount given known values (KH and PH).  In that case, you're not necessarily looking for a shift, but you're trying to read and calculate based on the chart. 

Use one method, not both.  I'm sure everyone has their preference on which is easiest to understand as well.

[Mmiller2001]

The KH/pH chart does not account for other acids in the tank (such as nitrification), No3 is a weak acid as an example. Given no other acid sources, then the chart is accurate but we know this is not so simple; which is why you are seeing extreme CO2 levels on the chart The better method is using pH drop to measure CO2 (I don't believe Farmer developed this method). You will want to take a sample of the tank water and let it sit open for 48 hours. Take a pH of this sample and this will be your degassed pH (the baseline pH). While injecting CO2, you will want a 1 to 1.5 pH drop from the baseline pH. You must observe the tank, while making this adjustment for any signs of stress. If you see signs of stress, then back CO2 down just a bit and you will have reached optimal CO2.

My tank's degassed pH is about 6.5, my tank high pH is 6.0 and I drop pH to 5.05 and 4.97 stresses the fish.

[Jeff_F]

Hi  @Mmiller2001,

  My degassed water (water after sitting out for 48 hours) pH is 7.3.  My pH in the middle of the night after the co2 has been off for 10 hours or so is 6.75 and my pH at about 2:30 in the afternoon after going on at 9:00 in the morning is 6.1.   So, what would you say my co2 ppm is?  Slightly more than 30ppm?  The drop checker is a light green color and I have good plant growth and the fish look fine.  The fish do not show any issues.  The water temperature is 76 degrees.

[Mmiller2001]

On 9/2/2023 at 12:24 PM, Jeff_F said:

Hi  @Mmiller2001,

  My degassed water (water after sitting out for 48 hours) pH is 7.3.  My pH in the middle of the night after the co2 has been off for 10 hours or so is 6.75 and my pH at about 2:30 in the afternoon after going on at 9:00 in the morning is 6.1.   So, what would you say my co2 ppm is?  Slightly more than 30ppm?  The drop checker is a light green color and I have good plant growth and the fish look fine.  The fish do not show any issues.  The water temperature is 76 degrees.

Very close to 30ppm. That’s a 1.2 pH drop. You’re in a good range.