GreggZ
Member
Yes the relationship is not linear.
But that’s only the beginning. Next is exploring methodology. Folks will often list tank parameters and say their CO2 is at 30 ppm. But is it? Best answer is “maybe”.
There are loads of ways to introduce errors into that equation. Let’s start with measuring dKH. Let’s take an example where someone measures their dKH at 5 and measures their fully degassed pH at 7.6. That would indicate a CO2 concentration of 3.78 which would be reasonable at equilibrium with the atmosphere. So they drop pH to 6.6 and claim they have 30 ppm CO2. The first mistake is that fully degassed water already has some CO2 in it so that actually calculates to 37.8 CO2 ppm..
But let’s say their measurement is off. 99% of folks use a liquid drop kit to measure dKH. If the true dKH is actually 5.7 CO2 would now calculate to 43.09. If the true dKH is 4.1 then CO2 concentration is 31 ppm. Either would be easy to do as we are not measuring down to tenths.
So there’s one variable that can have an effect on how we report CO2 concentration. Next is the methodology of measuring pH. Some folks use test strips or liquid kits that can be off by quite a bit. Using the same example above let’s say that that instead that instead of degassed pH at 7.6 it’s really 7.4 and instead of dropping to 6.6 it’s really 6.8. This is extremely easy to do when using a test kit and deciphering shades of color. Now CO2 concentration would calculate to 23.85.
And it’s even true when using calibrated probes. A long time ago I tested 3 different calibrated pen type pH meters against each other. They all gave different readings. Next best is a higher quality BNC connected type probe. More accurate but still not lab grade equipment.
So the point is many times the reality is that measuring pH drop and dKH are a garbage in garbage out methodology. And when you combine errors in both readings you can begin to see how two people testing the same water could come to wildly different conclusions about their CO2 ppm. So when someone says my CO2 is at 30 ppm, most times they really have no idea if that is true or not.
Then you can also bring in other variables which can affect pH. Our tank is not a lab. There are many other forces at play. Source water dKH can change over time, even seasonally in many places. Some municipalities inject things like Sodium Hydroxide to alter pH. And the list goes on. Even a difference in plant mass and CO2 uptake can throw things off.
So then the question is why does it matter? In my experience optimizing CO2 makes every single other thing easier. If you get CO2 right, you have much more leeway as to nutrient dosing, as a wider range will still produce very good results. Folks love to discuss dosing down to the smallest detail. This ratio vs that ratio. Lean vs rich. K must be below NO3. PO4 must be limited. And the list goes on and on. In my experience if you take the time to get CO2 right, these topics become far less important.
So if we can’t trust the CO2 ppm calculation, how do we dial in CO2? First is having a reliably constant flow rate of CO2. Many cheap needle valves drift. Counting bubbles accurately is all but impossible and bubbles are not a uniform measurement. I use a flow meter and even that can have limitations. So the odds of someone’s CO2 ppm being constant is low.
For me the best method is trial and error and patience. Closely observe both fish and plants while dialing in CO2 slowly over a period of time. Plants will provide subtle clues as you get closer to your optimum concentration. If fish show distress time to back it off just slightly. And one thing many don’t consider is that O2 and CO2 are not mutually exclusive. That is you can have and want high levels of each. If you have good surface agitation (O2) you can have higher levels of CO2 without affecting livestock.
I can tell you this. If I see something wonky in my tank, the first thing I do is check and double check CO2 levels. If it’s off I could waste a lot time playing whack-a-mole with ferts when the real issue is something completely different.
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