# Tank Oxygenation: Plants v. Aeration



## dwalstad (Apr 14, 2006)

Recently, a fishkeeper sent me a tank photo that inspired this thread. The photo shows vigorous water oxygenation via a HOB spillway filter in one tank and a whole bunch of air stones in another. That would be okay but for the fact that the tanks contain live plants!

Fishkeeper’s Question: Thank you for explaining the need of CO2 for plant growth. The reason I put in the bubblers was because a lot of people on Facebook groups told me that it's important to add oxygen if I'm dosing medication or fish just needs more oxygen in general. Can you please help me to understand the difference?

My Response: Fish need oxygen but air-water mixing like this is IMHO excessive and unnecessary. The effect of the HOB spillway filter is worsened by the fact that the tank water is so low. And it looks like there’s already another pump/filter in the tank!

Excessive air-water mixing like this can exceed (saturate) the oxygen requirements of fish. This would be harmless in a “fish-only” tank, but in a planted tank, it deprives plants of carbon. (Plants need carbon more than any other nutrient, and water CO2 is typically scarce in tanks without artificial CO2 injection.) It ignores the fact that plants produce oxygen. In these tanks, plants will never grow fast enough to prove their worth, in terms of oxygen production and ammonia uptake. 

I have 9 tanks without aerators or filters. True, the tanks are shallow (12 inches deep), so they have a big surface area that enhances air oxygen diffusion. For deeper tanks with less surface area, I recommend a submerged pump or very gentle air bubbling to circulate water. In the fishkeeper’s tank with the HOB, I’ll bet that the one submerged pump/filter (left in photo) would be sufficient for the tank.

However, all ecosystems are different and you may have to adjust--the art of fish and plant keeping. You can gauge oxygen levels by fish behavior. If the fish are listless and hanging at the surface, particularly in the early morning, then of course, you must increase oxygenation. If you raise bottom-dwelling fish, you will have to cater to their particular needs.

However, at some point, you should be able to find a balance between optimal oxygenation for fish and conserving precious CO2 for plants. But “air-blasting” a tank containing live plants like this is counter-productive. Better that the plants were plastic or not there at all!


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## mistergreen (Mar 3, 2007)

I have a jungle tank. At night, my air pump turns on to provide a little O2 for the fish and plants. Plants use O2 at night and having so many plants, they could compete with the fishes. I inject CO2 so I don't have to worry about Co2 generated by the soil. 

I don't know if it makes a difference but it couldn't hurt.


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## dwalstad (Apr 14, 2006)

mistergreen said:


> At night, my air pump turns on to provide a little O2 for the fish and plants. Plants use O2 at night and having so many plants, they could compete with the fishes.


Timing is everything!


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## mysiak (Jan 17, 2018)

I have 2 jungle tanks, both extremely overstocked (when looking at the usual recommendations - I call it "natural maximum tank capacity"), 16l nano is basically ignorant to extra aeriation. Plants or fish don't care. I run small air stone in the filter compartment just for the ease of my mind.

However my other 160l tank reacts very positively to heavy aeriation - I run venturi aerator on the filter output + additional air stone (mainly as a backup, should anything happen to the filter). Fish were sometimes gasping at the surface in the morning without it. But what surprised me the most are plants - they prefer when I run aeriation 24/7. They grow better, with no/less algae as compared to no extra oxygen, or when I run the aeriation only during night. I can't really explain why plants react this way - I expected them to be happier with extra CO2. I tried to limit the addition of extra oxygen/air exchange a couple of times, but I always return to the non-stop schedule. Maybe the plants were limited by nutrients (I have to dose iron and potassium daily even now) instead of CO2? Who knows.. 

Anyway, long story short, sometimes even counterintuitive actions result in positive reaction.


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## dwalstad (Apr 14, 2006)

Love your post, because it shows the complexity of aquatic ecosystems. You have the knowledge to do intelligent tweaking, whereas beginners may not. A small air-stone in the corner could be useful. It circulates water bringing nutrients to the plants. Oxygenation also stimulates decomposition and decomposition releases CO2 and other nutrients that plants can use.
Because there are no pat answers, I always try to leave _wiggle room_ in my recommendations.


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## dwalstad (Apr 14, 2006)

mysiak said:


> I have 2 jungle tanks, both extremely overstocked (when looking at the usual recommendations - I call it "natural maximum tank capacity"), 16l nano is basically ignorant to extra aeriation. Plants or fish don't care. I run small air stone in the filter compartment just for the ease of my mind.
> 
> However my other 160l tank reacts very positively to heavy aeriation - I run venturi aerator on the filter output + additional air stone (mainly as a backup, should anything happen to the filter). Fish were sometimes gasping at the surface in the morning without it. But what surprised me the most are plants - they prefer when I run aeriation 24/7. They grow better, with no/less algae as compared to no extra oxygen, or when I run the aeriation only during night. I can't really explain why plants react this way - I expected them to be happier with extra CO2. I tried to limit the addition of extra oxygen/air exchange a couple of times, but I always return to the non-stop schedule. Maybe the plants were limited by nutrients (I have to dose iron and potassium daily even now) instead of CO2? Who knows..


Mysiak,
I wonder if you would be willing to post a picture of your 160 liter tank, the one where the plants responded positively to aeration. I would like to write an article for my aquarium club's newsletter (Raleigh Aquarium Society) on tank oxygenation. It would be nice to include your information and a photo along with what I first posted. A photo would also show people here (APC) what an experienced aquarist can do!  Glad to see you back posting on the forum.


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## mysiak (Jan 17, 2018)

Hard to capture the jungle in photos, but hope you get the idea. 

More info about the tank can be found in this thread - 180l (~47gal) tank (I mix between 160/180l, officially it's 180l, but should contain ~160l of water as it's not supposed to be filled up to the rim).


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## dwalstad (Apr 14, 2006)

Mysiak, thank you so much for posting the pictures! The amount of fish (~400) and plants stuffed into this 47 gal tank is amazing. 

With this many fish, I can see why aeration would be critical for fish survival. The many fish constantly generating CO2 by their respiration provides plenty of CO2 for plants. Aeration has apparently also benefited plants by stimulating decomposition of water DOC, thereby "unlocking" DOC nutrients--including CO2.

I went through your thread starting in 2018 and could not find any reference to a filter. 

Question: Does this tank have a filter?


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## mysiak (Jan 17, 2018)

Thank you for providing a possible explanation of my observations, it makes sense now. 

Yes, this tank has stock filter which looks like this (it is located in the right corner). It has an unusual design decision - "polishing" filter floss is located as the very first media, so it captures majority of all the waste before entering the filter. Floss has to be replaced every week, but on the positive note, the rest of the sponges basically never clog and don't need any cleaning whatsoever (except the motor/propeller every ~3 months). I'm not really sure if it does any good in terms of biological filtration (other filter media look almost like new), but as the filter's glued in and acts a mechanical filter + moves water around, I keep using it. 

I ran the filter without the filter floss for some time, but it was hard to predict when the filter is it going to clog completely and it was pain to clean everything so I abandoned the idea eventually. Initially I thought that this design is just a "cash grab", but I actually like the idea now (I'm buying generic filter floss in bulk).


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## Steven F (Aug 1, 2021)

Any water movement that adds oxygen to the water will also add CO2. Also the fish take in oxygen and exhale CO2. If oxygen and or CO2 is at saturation increased water movement will allow these gases to escape the water.

The myth that increased water movement drives out CO2 is based on high tech tanks with CO2 injection. A device called a drop checker is used to verify the tank has enough CO2. The drop checker only detects CO2 when it is outgassing from the water. It shows green. When water is not outgassing CO2 it shows blue. If you remove a drop checker from the water it always shows blue even though the air has 400ppm of CO2 in it. If you add CO2 to a dry box and a drop checker (CO2 level over 400ppm) the drop checker shows green (CO2 OK) or yellow (CO2 too high). So in high tech tanks CO2 is always maintained at a level over 400ppm. 

Now in high tech tanks people have found that too much surface water movement causes CO2 to drop checker to read too low. Reducing surface water movement would then cause the drop checker to read OK. So people always assume more water movement is bad for CO2 levels in the water. I have not seen any evidence to support that belief.

Now if you have passive CO2 setup the drop checker will never show green (OK) it always show blue. I have found passive CO2 works very well and changing surface water movement never cause any noticable effect on plant growth.


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## dwalstad (Apr 14, 2006)

I don't see how you can get around the fact that water in equilibrium with air contains a fixed and rather low CO2 concentration--in terms of what plants require for good growth. Air by itself provides very little CO2 for plants.

Water in equilibrium with air contains about 0.5 mg/l CO2 at room temperature [Textbook reference: RG Wetzel (Limnology, 3rd Edition, p. 188)]. Global warming has increased air CO2 from 360 ppm (year 2000) to about 410 ppm (2020), which is a 14% increase. So you could say that water CO2 in equilibrium with air has increased 14% now to 0.7 mg/l.

Water mixing or aeration will speed up gas exchange and equilibration. If the tank water has CO2 levels below 0.7 mg/l, air-water mixing will increase the CO2 level to 0.7 mg/l--but nothing beyond that. How could it? If water CO2 levels are above 0.7 mg/l, water mixing and aeration will hasten the degassing process to bring water CO2 levels back down to the 0.7 mg/l equilibration point.

Decomposition by bacteria and respiration by fish can bring CO2 levels up above the measly 0.7 mg/l from the air input. I have measured 2-10 mg/l CO2 in my tanks due to these natural processes (my book, p.179). Air-water mixing will hasten the removal of this precious CO2 to bring level down to 0.7 mg/.

That said, air-water in some instances may be helpful. Aeration can increase water CO2 levels by stimulating decomposition by bacteria. Or water movement can bring nutrients to plants faster and increase their growth rate. 

I started this thread with what I consider to be over-zealous aeration by a beginning hobbyist. My advice applies to that situation.


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## day (6 mo ago)

I'm so glad to see this discussion. I have only fairly recently begun (again) using co2, & found, then lost, a very scientific explanation of why aeration (done correctly) actually helps with co2 dispersion in the water. Unfortunately, I never finished the article before losing it, but will try to find it again & post back. I'm still in the experimental stage of using co2, still with a diy citric acid/bkng soda generator, but want to learn more before investing in a better system. So far it's been fun to watch the plants thriving so much better. When I did it last, several years ago, I did it in a plant-only tank & used the sugar/yeast formula for the generator, & had zero problems. This time it hasn't gone so smoothly, but after a few months I think I've got it figured out better. Anyway, I will post if/when I find that article again. Best to all here.


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## johnwesley0 (Feb 23, 2021)

I'm having to reevaluate what to do next with my apistogramma walstad tank as it enters the half-year mark. I was so worried about setting up my system in time for the arrival of some mail-ordered fish that I feel as though I may have over-done it in terms of emergent plants with the "aerial advantage". Today when I look at them, my lucky bamboo and pothos are dark green and ascendant while my rooted dwarf sag look a bit exhausted. Hair algae has also begun to appear in earnest.

I'm wondering whether it's time to start removing some of the terrestrial plants to the extent that they may be starving my chief only oxygenators of nutrients?

OR, should I just add more fish?


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## tiger15 (Apr 9, 2017)

dwalstad said:


> Water in equilibrium with air contains about 0.5 mg/l CO2 at room temperature [Textbook reference: RG Wetzel (Limnology, 3rd Edition, p. 188)]. Global warming has increased air CO2 from 360 ppm (year 2000) to about 410 ppm (2020), which is a 14% increase. So you could say that water CO2 in equilibrium with air has increased 14% now to 0.7 mg/l.
> 
> Decomposition by bacteria and respiration by fish can bring CO2 levels up above the measly 0.7 mg/l from the air input. I have measured 2-10 mg/l CO2 in my tanks due to these natural processes (my book, p.179). Air-water mixing will hasten the removal of this precious CO2 to bring level down to 0.7 mg/.


I think 0.5 mg/l CO2 is for outdoor equilibrium. Indoor CO2 is higher than outdoor, more so in winter months, better insulated home, gas heat and stove homes.

Are your 2-10 mg/l CO2 measurements based on pre photo period. In my window sill planted bowls, equilibrium CO2 is stripped to < 0.5 mg/l by photosynthesis under direct sunlight, so aeration to replenish CO2 helps.


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## day (6 mo ago)

So, this is something with which I'm sure many/most of you are already familiar, but I wrote that I'd try to find the article I'd lost (before reading entirely) explaining the need for aeration with co2 injection. Never could find it, but did find this website, which explains it about the same way as the other. I had already labored over understanding the beginning parts, so skipped to the "3 tools" part at the bottom, & am now wondering how to do this, since I have the co2 being injected directly into the HOB. I'm not sure if I should have an airstone on the other side or just get a spray bar or a surface skimmer. I'll dig deeper, but for now, the plants are enjoying it. Anyone who might comment on this, I'd appreciate it. I'm too busy to get seriously technical about it all right now. Surface agitation & gaseous exchange in CO2 injected tanks


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## mistergreen (Mar 3, 2007)

tiger15 said:


> I think 0.5 mg/l CO2 is for outdoor equilibrium. Indoor CO2 is higher than outdoor, more so in winter months, better insulated home, gas heat and stove homes.


Indoor CO2 can bring it up to 3-4mg/L. I've measured CO2 up to 14mg/L in a dirt tank with fish. If added aeration, it would bring the CO2 back down to ambient levels, 3-4mg/L.


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## dwalstad (Apr 14, 2006)

johnwesley0 said:


> I'm having to reevaluate what to do next with my apistogramma walstad tank as it enters the half-year mark. I was so worried about setting up my system in time for the arrival of some mail-ordered fish that I feel as though I may have over-done it in terms of emergent plants with the "aerial advantage". Today when I look at them, my lucky bamboo and pothos are dark green and ascendant while my rooted dwarf sag look a bit exhausted. Hair algae has also begun to appear in earnest.
> 
> I'm wondering whether it's time to start removing some of the terrestrial plants to the extent that they may be starving my chief only oxygenators of nutrients?
> 
> OR, should I just add more fish?


Hard to say. The terrestrials and algae must be getting most of the nutrients and light in this setup.
Perhaps removing some of the terrestrials and adding root tabs for the Sag? (Adding more fish might just make your terrestrials and algae grow even better with no help for the Sags.)


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## dwalstad (Apr 14, 2006)

Second thoughts....

If Mysiak, mistergreen, and other observant hobbyists are getting better plant growth with added aeration, I propose--after some reference work--an explanation.

In stagnant water, heavy photosynthesis would deplete CO2. It would also fill area surrounding leaves with unwanted oxygen. (Excess oxygen can reduce photosynthetic efficiency in some plants up to 50%.)

Outdoor air contains 350-450 ppm CO2 and indoor air contains more, about 1,000 ppm before it reaches unhealthy levels for humans. So water in equilibrium with indoor air probably contains more than the standard 0.5 and 0.7 mg/l that I quoted, let's say 1.0 mg/l. Not much CO2 but some. (Mistergreen's 3-4 mg/l is a little more than I would predict but maybe possible for those who live in sealed houses, lots of inhabitants, and gas stoves?)

Plants in stagnant water with *zero* CO2 (depleted by heavy photosynthesis) would probably benefit from water-air mixing that would constantly bring them 1 mg/l air CO2 and quickly flush out the inhibitory oxygen. The associated water movement would decrease the leaf boundary layer speeding up both these beneficial processes. 

Aquatic botanists and my book recommend moderate water movement, but I always thought that water-air mixing (e.g., bubbling and degassing) would be detrimental. However, I realize now that in some situations aeration actually could be beneficial.


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## mistergreen (Mar 3, 2007)

dwalstad said:


> Mistergreen's 3-4 mg/l is a little more than I would predict but maybe possible


Yeah, in a small bedroom where big lumbering human mammal sleep, produced an unhealthy level of CO2. I think I measured exhaled levels to about 7000ppm+ of co2. A room level can be 2700ppm. It’s good to ventilate every day especially if you‘re adding CO2 tanks. I fan out the house every day.
I have a CO2 sensor if you’re wondering how I got these numbers.


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## Michael (Jul 20, 2010)

I use hang on back filters on all of my tanks except the 75 which has a cannister. All the tanks are mature Walstad systems, even the youngest tank is several years old. I arrange the filter return and water level so that water flows smoothly into the tank. There is never a waterfall or splashing. I do not use air stones or other types of aeration. I only use mechanical media in the filters.

Confession: I often keep too many fish in my aquaria. The HOBs keep oxygen levels high enough for the fish when plants are not photosynthesizing. Being at the mercy of the Texas electrical grid, frequent power failures which stop circulation AND photosynthesis can kill fish.

Where do these tanks fall in the spectrum of CO2 levels? Are they constantly at equilibrium with atmospheric CO2? Does natural CO2 build up during darkness (including siesta) to be used by plants when the lights come on? My fish are healthy (except during power failures) and plants show consistent moderate growth. Algae is never a serious problem.


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## dwalstad (Apr 14, 2006)

This is very helpful information. Thanks!


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## tiger15 (Apr 9, 2017)

My window sill planted bowl has measured pH fluctuated from 7.2 to 8.8 pre and post sunlight period. With measured kH around 3, the calculated CO2 is between 3.6 to 0.1 ppm. So Mistergreen's 3-4 mg/l is within range of mine, noting that this is more than atmospheric equilibrium CO2 due to respiration from live stock and bacteria. Note that the water is stagnant and floaters covered 90% of the surface, so surface air exchange is limited. I was afraid there won't be enough O2 exchange, but apparently vigorous O2 streaming during sunlight period generates adequate O2 to carry through day and night. 

Homes on gas heat and stove contribute additional CO2. I have a CO monitor but no CO2 monitor to know what CO2 level I have. Elevated CO2 can make people sleepy, and extremely high CO2 can kill by asphyxiation. I think the safety threshold is 3000 ppm for CO2 operated greenhouses. 

Indoor air quality is typically worse than outdoor, and better insulated house is worse due to accumulation of CO2 and other indoor pollutants. The worst time is returning home from vacation when normal door opening and closing to exchange fresh air has been absent. It's good to grow house plants and planted tanks for air purification.


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## mistergreen (Mar 3, 2007)

Aquarium CO2 systems add extra CO2 in the house as well, equivalent to the CO2 operated greenhouses.


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## day (6 mo ago)

dwalstad said:


> Second thoughts....
> 
> If Mysiak, mistergreen, and other observant hobbyists are getting better plant growth with added aeration, I propose--after some reference work--an explanation.
> 
> ...


I need to get your book! I'm still so green (n.p.i.) at all of this, & too busy w/too many other things to do all I want with the aquariums, but after only 3 years in now, keeping fish & wanting to do it right, plus being so admittedly ignorant, I'm glad we have your input available here. So glad! That you yourself continue to research & learn, & then share, is a testimony to your humility as an undeniable & admirable expert. Thank you for everything!!


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## PlentyCoup (Aug 22, 2021)

This was a great read as usual, and gave me a lot of insight as I am figuring how to set up my Ehime 250 on my 36gal. I would never not have water movement on a tank of that size but was prepared to make adjustments to the sprayer bar based on how things progressed and the fact that the general concensus is no or very little water flow and absolutely no filter media. So reading about filters in use with various media at least mechanical is great to hear. 
I am thinking of removing the filter from my betta tank but keeping the little bad of mechanical media. 

I can verify on Diana's comment on about stagnant water. At one point the pump failed on my smaller pond. I didnt have any fish in it at the time so i figured everything would be ok. They were.... sort of. All of the pond plant with aerial advantage kept chugging along. The strictly underwater plants (cant remember what varieties i had at the time) saw significant slowing in growth, some stopped growing altogether without the pump running. It was small 80-100 gallons so i am figuring this small size amplified the issues i was seeing that might be seen in larger bodies of water. It was probably 6 years ago when all this happened and i think i salvaged what i could into tubs with simple bubblers and most of the pond plants rebounded.


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