CO2 – best practices, myths and unknown territory

[Yugang]

We have 3 decades’ worth of CO2 knowledge on several fora, and the one thing that stands out from thousands of posts is Tom Bar @plantbrain saying a long time ago that 80% of problems in tanks that he sees are CO2 related. As we continue to learn, we have some generally accepted CO2 best practices and better tools for CO2. Do we have consensus as to what is ‘good CO2 for the plants’, and what are some myths worth debunking or areas worth further exploring?

I’ll kick of this thread with my understanding, and the questions that I have, but when it comes to how a plants and a planted tank experience CO2 I am not the best expert. I hope that others chime in, with experience but also challenging some myths.

---------------------------------------


Best practices (Yugang’s version)

• Generally it is recommended to target 30 ppm CO2 for high tech tanks. This will correspond to lime green in a 4 dKH drop checker and a 1.0 pH drop from fully outgassed tank water.

• Generally it is recommended to make sure CO2 is stable during the lights-on photo period. With a pH probe (pH being a proxy for CO2 ppm) a maximum variation of 0.1 pH is recommended, within the photo period of one day, but also and especially from day to day and longer term. Plants adjust to any CO2 ppm as long as it is stable, but would need to invest significant energy adjusting to changes. Decreasing CO2 ppm is (far) more detrimental to the plants health than increasing CO2 ppm, and stability is the only truly good reading.

• It is recommended to have a good and gentle flow anywhere in the tank, without any dead spots, to enable CO2 transport to the plants anywhere in the tank. While measuring with a pH probe, different readings in different locations in the tank is a red flag.

• It is recommended to have good surface agitation for gas exchange

Myths (Yugang’s version)

• Supplement CO2 to the planted tank is beneficial

Well, it depends. Applied well, with good skills, CO2 can be beneficial. Otherwise, many hobbyists would enjoy more sustainable success with a low tech tank and no CO2 supplemented.

• The planted tank world can be divided in high tech and low tech

Probably it makes more sense to see this as a continuum, from non-CO2-supplemented, to lightly CO2 supplemented, to high tech full supplemented. It seems likely that any choice of CO2 ppm on this continuum has some benefits as well as some disadvantages.

• The higher CO2 ppm the better.

This is probably a myth, and depends on personal objectives and other tank parameters (temperature, plant species, light, ferts) as well. My personal take is that CO2 stability (hours, days, weeks) is far more important than whether we target 20, 30 or 40 ppm.

• Unstable CO2 causes Black Beard Algae.

This is probably a myth. From my experience, and my working hypothesis, unstable CO2 causes plants to suffer, potentially die. CO2 going down is more dangerous than it going up. The combination of poor tank maintenance, organic waste and the suffering plants is the true cause of BBA . This means in my experience that a well maintained tank is has more leeway for CO2 imperfections, than a tank with waste organics and less than perfectly healthy plants.



Unknown territory (Yugang’s version)

• What is the rationale, what are the trade offs, for choosing a certain CO2 injection rate?

While 30 ppm CO2 (lime green drop checker, 1.0 pH drop) is the “middle of the road” recommendation for high tech beginners, we see several experience scapers going to 40 ppm and higher. Some of them may want to farm plants for sales, achieve quick success and ready for a new scape, or have extremely demanding species. I don’t know of any guidance when I should pick 20 ppm as my tank’s stabilised CO2 level, when 30 ppm would be better than that, or perhaps push it to pH drops as high as 1.6? Lower ppm injections will greatly save on consumption, be more comfortable for livestock, and will save a lot of time pruning too fast growing plants. What would be the best decision parameters to pick the optimum CO2 target for a tank?

• Targeting anywhere on 5 ppm – 10 – 15 – 20 ppm CO2 injection

This is the rarely used domain between low tech and ‘middle of the road’ 30 ppm high tech. Very few can create a low tech tank look lush and colourful like high tech, but how much easier would it be to achieve similar results (especially at higher temperatures) by injecting just a little, without going full high tech?

• When aiming to create good conditions (shape, colour, growth) for 90% of the frequently available plants for the hobby, what CO2 ppm would be sufficient?

I am not sure, but would estimate that 20 ppm CO2 is more than sufficient, and saves on CO2 consumption and maintenance.

• Indicator plants for CO2 instabilities

I am using Rotala wallichii, that stunts immediately.

• (semi-) Closed high tech tanks

I have been experimenting with this, it is quite feasible to save perhaps up to 80% on CO2 consumption, while having superior stability throughout the tank.

 

[plantbrain]

How are you measuring CO2? Drop checker which is nigh worthless for decent control and measurement sure is a big assumption. I have never recommended them in over 30 years. Basically I have never suggested aquarist use them. Good luck getting + or - 10-20 ppm accuracy trying to read those colors. PH meters and probes are far superior. Those can be improved upon as well and used in conjunction with a CO2 probe which gives another reference, addresses KH and gives a delay range between the two probe types. If you want to talk measurements, you need to be able to make them critically.

Temp, general care are oft overlooked issues that play into ranges of growth rates. 20 C is radically different from 30C. Thus your idea of using less gas will work much better at lower temps and you’ll have higher O2 also. General care is likely the biggest problem for hobbyist. Arms in the tank. Makes a massive difference in success and adjustments.

Light was also not discussed but hey, it’s only one post, I get it. Hard to cover all the things that causes so many mistakes by aquarist, but lots of light or high light disease (HLD) is prevalent in the hobby. LED lighting has made it worse. As did T5 and PC bulbs before. MH are rarely used these days. More light = more growth which means more and more CO2.

So lower light, 50 umols, lots of algae eaters, rich CO2 and cool tanks will give robust but easy to care for planted aquariums. Add good water changes and care, trimming, filter clean, maintain constant water levels for evaporation, you can avoid water changes for long periods and slower easier to maintain growth.

While folks talk ppm of CO2 these are secondary ranges, ones we use to get close. Then we watch and slowly and incrementally adjust the gas up till there is no benefit for the plants. That takes skill and insurance that everything else is covered when doing this….i add shrimp at this point. Fish about 3-6 weeks later. Never had any issues with gassing them at the max CO2 ranges for plants.

THEN I go back and measure CO2 as best I possibly can. I measure gas in mls/second or minute. Next I pull out the CO2 probe. Check it against pH probe. Run both and see if there’s any differences. I come in routinely over a wide range. 40 ppm for a lower light tank, to 70 ppm for other higher light tanks in 100-150 umol ranges and warm temps in the 32 C ranges full fairly touchy fish. They bred as did the shrimp.

This tells me a few things.

I have done my due diligence more than anyone else that’s debated the subject to date.

I test only where I have success.
This falsifies the claim those successful ranges are somehow bad or cause x, y or z. It’s the applied scientific method in fact. The hypothesis must be falsifiable and if not, the issue is speculation and claiming to know more than evidence actually supports.

Rhetoric does not support claims nor does correlation. Wishful thinking does either. There are many fallacies aquarist, ( we all have at one time or another have made,) have done and each cohort that comes along seems they must do this as well, over and over.

That is fine. It’s how we learn.

Main thing is to use the principle of falsification to rule things out. Maybe you cannot rule out a lot of things yet. It’s hard to use if you lack control or do not yet understand the role CO2 plays fully. Took me a long time also. Took Amano even longer. But we both it could be done. 3 years for me, 10 years for Amano. I admired him for suffering for a decade with BBA . I was at the end of my rope. I could not have made it 10 years.

Point is to think through them. Do not assume. Adjust till the plants are doing really well. Then go back and measure and do a good careful job of it. There is your data. Think what might increase the rates or lower the rates of growth, make notes of those things also.

We’d expect to see wide ranges of gas. I would suggest a higher rate than 80%, closer to 90-95% for the root causes for CO2. But indirect issues like general care, flow, temps, lighting, how the CO2 is measured cause a lot of issues. These are all related to the gas however and part of the CO2 issue. Folks need to rule those factors out before hand (and…they can do those things) .then look at the gas. Then you have a very different opinion and view. And that’s not a bad thing.

 

[Yugang]

Thank you for your insights @plantbrain , there are some very deep truths in what you write. It reflects a huge experience. I read your post now several times, and still learning and digesting.

I test only where I have success.

This is my favourite quote, a lot to unpack from these words as it truly focusses on the plants.

I have several questions, but I'll restrain myself

 

[GreggZ]

THEN I go back and measure CO2 as best I possibly can.

This is also where errors come into play.

People say "I have 30 ppm CO2" with great confidence. The truth is they really have no idea. It's more of a general ball park.

There can be errors in reading dKH , and in general dKH readings are not precise. Is that 5 dKH really 4.1? Or 5.9? Who knows. But it throws off the calculation quite a bit.

Then folks use poor pH meters. Those cheap pens are notoriously off even when kept properly and calibrated. And test strips are worse. So is the 6.6 pH reading really 6.6? Or is 6.3? Or 6.9?

Throw all that into a calculator and you can get wildly different results. It's easy to see how people get it wrong.

If I put my numbers into a calculator it would indicate my CO2 concentration is about 90 ppm. Is it really? I doubt it. But makes little difference. I dial in CO2 slowly keeping a close eye and plants and fish. When the fish show signs of stress I back off just a little and that is my pH drop target.

It's the same for other things we measure in our tanks. It's not really the absolute value that matters, it's the relative value. And finding them always takes some trial and error and effort.

 

[Yugang]

Note sure if it is really helpful, but this is one of my thoughts on the discussion on CO2 measurements and optimising CO2 for the tank.



I read @plantbrain post several times, and with my modest skills I won't yet be able to replicate perfectly what he does. I could aim for it, sure. Same for @GreggZ , these are two who operate at the highest levels, take a fundamentally holistic approach and know how to read the plants. At their level they will question any measurement, because they understand the limitations of technology, and they know that one measurement is just a small part of the holistic picture.

When I started with high tech after a long absence from the hobby, I had no idea how importance CO2 stability and consistency are. I didn't even understand the value of measurements and was "unconscious incompetent". After several years of experimenting and measuring, and many mistakes, I am now probably "conscious incompetent" when it comes to CO2.

I agree that drop checkers have limitations, but when I see a tank without drop checker it is probably more likely that the tanker doesn't understand what it is supposed to do, than that it is someone at @plantbrain or @GreggZ competence level. Same for pH probes, I believe that despite the caveats it would be a huge benefit if more people would actually measure their pH profiles -- know if CO2 stability is acceptable, or a likely problem area.

Could a similar argument could be made for various other topics, including balancing light with CO2, and BBA ? The take aways for the "unconscious incompetent" may be different from the "conscious competent" pushing to the highest level? Is what the hobbyist at the highest level understand and practice, also best advice to beginners?

 

[Art]

Thanks @Yugang for the thought-provoking post. The title refers best practices, myths and unknowns. I'll add some of my learnings as well on these three points. At the very least, it's how I do it to maintain a tank that is not at the @plantbrain or @GreggZ level but is just as good as many out there.

Best practices

• Add supplemental CO2. IME, very few people are OK to keep a slow-growing plant tank unless they it really is a fish-focused tank like discus. Sooner or later, they will want more demanding plants and they will do better with CO2. Even slow-growing plants will do better with CO2 added.
• As you mentioned, as important is CO2 stability. I focus on a consistent 1.4 pH drop. Like @plantbrain, I don't use or recommend drop checkers as they cause confusion. Who hasn't read posts like "Can someone help me determine if this color is green, green/yellow, or yellow?"
• An internal circulation pump that ensures even flow throughout most of the aquarium.
• The best way to determine if you have enough is by watching your plants and balance that with fauna (fish, shrimp). Don't worry about finding a magic ppm number.
• If you use a pH meter, make sure to calibrate it often and double check it. I have an Apex pH probe that I calibrate about once a month. I then double check its accuracy with a quality pH pen that I also calibrate.

Myths

• Like you, I believe CO2 instability or insufficiency is just one factor that adds to the risk of algae growth. It isn't the root cause of algae. Frankly, I feel that it's the interruption in healthy plant growth that opens the door for algae to thrive.
• The need to hit a magic number of 30 ppm for plants to be healthy and algae to go away.
• Fish and shrimp can't live in such a high CO2 environment.
• How can you supplement CO2 with no KH in the tank! The dreaded pH crash!

Unknowns

• Like almost everything in this hobby, I'm sure there is a lot we still don't know. We just don't know what we don't know.
• We have not all agreed on what the best CO2 delivery method is. Perhaps we don't need to reach that conclusion.
• How does one supplement CO2 without measuring pH? Can you reach optimal state using a drop checker?
• Will CO2 probes every come into the mainstream as hobby-grade products?