Question of the Day Optimizing your CO2?

[Art]

OK let's do this!

You always hear advice from the more experienced folks, "You just need to dial-in your CO2". That sounds cool and, of course, it's true. However, what exactly does that mean??

So today's question of the day is, how (exactly) does one go about "dialing-in" the CO2 level for their tank? Be specific now. We are trying to document a subjective process here so we need to know the equipment that is used (it may be optional but helpful) and the process you go about to dial that CO2 in.

I know @plantbrain will well known to have one of the most "dialed-in" CO2 tanks around. I hope he chimes in with some nuggets.

 

[Quagulator]

My personal process is as follows:

1) Set up a tank and focus on the plants first and foremost. This way if I go way overkill on CO2 by accident, I won't all my fish.
2) Before I even turn CO2 on, let the tank sit for at least a day, filters running, lights off, ensuring there is rippling across all the water surface - measure pH after that period with a "decent" calibrated pH probe. I would recommend avoiding the liquid tests and at the minimum, use a pH probe that can be calibrated. If you have the means of using a high end probe, all the better. Ensure you store the probe properly.
3) The next day, turn CO2 on 2 hours before the lights come on. Use a flow meter if your tank is not considered a nano tank. Do a little research before hand - there is enough info on a google search these days to determine roughly how much CO2 flow is needed for your sized tank to achieve at least a 1.0 drop in pH from your initial measurement.
4) Measure pH again as the lights come on. If it's not at a 1.0 drop, increase the flow of gas slightly, and check again in an hour or two.
5) Do this for a few days, dialing in up to a 1.4 drop if you so choose. Some can get away with a little more or a little less.

After getting fish, try and be home for the first day or two so you can monitor them. If they are gasping at a 1.4 drop, dial the CO2 back or increase your surface agitation. You want to make 1 small change at a time here to find the limiting variable.

Eventually, to optimize CO2, you want to be have at least a 1.0 or greater drop in pH from your fully degassed water, but not so much that you are causing your fish to gasp at the surface. You want lot's of surface agitation - do not worry about "off gassing CO2" - if you want to optimize it, treat CO2 as if it's free.

Run CO2 at your determined drop in pH all the way up until the lights shut off.

Consider running an airstone at night if you are pushing the limits of your fish or if your surface agitation is less than ideal.

Do not make sudden, large changes in CO2 if you have livestock in the tank and will not be home to watch for results.

Use a drop checker for reference only - rely more so on the pH drop measurement (assuming your pH measuring capabilities are decent).

You do not need an exact measurement of the tanks actual pH, only a relative measurement of the pH drop.

Experience and knowledge will come the more you use CO2. CO2 makes everything more forgiving in a planted tank, low light, medium light and high light.

 

[GreggZ]

OK let's do this!

You always hear advice from the more experienced folks, "You just need to dial-in your CO2". That sounds cool and, of course, it's true. However, what exactly does that mean??

So today's question of the day is, how (exactly) does one go about "dialing-in" the CO2 level for their tank? Be specific now. We are trying to document a subjective process here so we need to know the equipment that is used (it may be optional but helpful) and the process you go about to dial that CO2 in.

I know @plantbrain will well known to have one of the most "dialed-in" CO2 tanks around. I hope he chimes in with some nuggets.

Great question Art. In my opinion there are many ways to get CO2 wrong. Folks will often list tank parameters and say their CO2 is at 30 ppm. But is it? Best answer is “maybe”. And the notion that we want to have 30 ppm Co2 is way oversimplified. In a high tech, high light high tank more is better. For instance in my tank I drop my pH by 1.4 daily via CO2 injection. The charts/calculators would indicate my CO2 level is about 100 ppm. Is it really? I doubt it.

There are loads of ways to introduce errors into the CO2/dKH 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 the CO2 concentration is 31 ppm. Either would be easy to do as we are not measuring dKH 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 liquid test kits 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. It’s best to use a higher quality BNC connected double junction type probe. Much more accurate but still not lab grade equipment.

One more issue is how people measure the full degassed value. Tanks do not fully degas overnight. It's best to leave a sample out for about three days. Then start testing pH about every 4 hours or so until the reading remain the same. I see people get their degassed reading wrong all the time.

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 many factors can affect CO2 concentration in the tank. As plant mass changes so does CO2 consumption (anyone ever notice how actank perks up when you massively trim a jungle?). Changes in surface agitation/aeration can effect CO2 levels. CO2 diffusers can clog and become less efficient. The flow rate of pumps/filters can change affecting CO2 dissolution. And the list goes on and on.

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. 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 or other things when the real issue is something completely different.