Surface agitation in low tech planted aquariums

[Kalita]

Hey all!

This is my first post on here. I thought it would be the perfect place to start a discussion on gas exchange and surface agitation in low-tech planted tanks. 

I am starting a 33 gal low tech and currently have 2 HOBs and a dual sponge filter running. As you can imagine, this provides a decent amount of surface agitation. What I am interested in learning about is the gas exchange of O2 and CO2 that will occur due to this agitation. Where I am not dosing with CO2 and it is required for healthy plants, I do not want to lose an abundance in exchange.

I did however watch the Circulation Q&A Corey posted where he stated that he uses air stones in his planted tanks for circulation purposes to help ensure nutrients are well dispersed in the water column. He touched on gas exchange there briefly, but I would appreciate a deeper discussion.

Tia to those more knowledgeable than I in this area! Looking forward to learning from you 🙂

Edited March 5, 2021 by Kalita

[Daniel]

The gas exchange happens naturally with or without surface agitation in a low tech aquarium, just like in lakes and ponds. There is enough CO2 in equilibrium with the CO2 naturally in the atmosphere to provide the normal requirements that aquatic plants need for photosynthesis.

When injecting CO2 in a high tech aquarium some people avoid surface agitation in order to use last molecule of CO2 but it is not something to be concerned about in a normal aquarium

[Dirtydave]

It is definitely not required to have CO2 to have healthy plants 90 percent of the time.Cory would be the first to say and has said it countless. I have no CO2 and my tanks are jungles.Have fun

 

[Kalita]

Oh I agree that CO2 injection (high tech) isn't necessary to maintain some healthy planted aquariums, however, CO2 is required for photosynthesis so it is needed in some amount. That amount just varies, depending on species, population, and desired growth rate. The plants that I'm looking at are all considered easy beginner plants that do well in low-moderate lights and low CO2 so not planning on injection.

I understand CO2 is produced naturally by fish, bacteria and some soils/fertilizer (or any decomposing organic matter e.g. excessive fish food) in the tank. Does it also enter at the surface via gas exchange between water and atmosphere? (Daniel - you spoke to this, but I'm still chasing clarity!)

I know gas exchange at the surface works by the process of diffusion, where high concentration flows to low in attempt to maintain equilibrium. I also understand that increased surface agitation allows for increased exchange rates. This explains why O2 levels are higher in tanks with greater surface area and agitation (Corey demonstrated this perfectly in one of his videos). Oxygen, however, is found in a great abundance relative to CO2 in the atmosphere (Nitrogen accounting for ~78%, followed by O2 at 21%. Comparatively, carbon dioxide is only present at ~0.04%). Given this information, would the concentration of CO2 be greater in the atmosphere or water?

Depends on the water, right? I know many high-tech planted aquarium keepers like low agitation to ensure they don't "gas off" the CO2 they are injecting, however there seems to be debate about whether surface agitation will increase or decrease the amount of CO2 in a low-tech planted aquarium. Anyone care to speak to that?

Edited March 16, 2021 by Kalita

[Daniel]

8 hours ago, Kalita said:

 

I understand CO2 is produced naturally by fish, bacteria and some soils/fertalizer (or any decomposing organic matter e.g. excessive fish food) in the tank. Does it also enter at the surface via gas exchange between water and atmosphere? (Daniel - you spoke to this, but I'm still chasing clarity!)

I know gas exchange at the surface works by the process of diffusion, where high concentration flows to low in attempt to maintain equilibrium. I also understand that increased surface agitation allows for increased exchange rates. This explains why O2 levels are higher in tanks with greater surface area and agitation (Corey demonstrated this perfectly in one of his videos). Oxygen, however, is found in a great abundance relative to CO2 in the atmosphere (Nitrogen accounting for ~78%, followed by O2 at 21%. Comparatively, carbon dioxide is only present at ~0.04%). Given this information, would the concentration of CO2 be greater in the atmosphere or water?

Depends on the water, right? I know many high-tech planted aquarium keepers like low agitation to ensure they don't "gas off" the CO2 they are injecting, however there seems to be debate about whether surface agitation will increase or decrease the amount of CO2 in a low-tech planted aquarium. Anyone care to speak to that?

Yes, CO2 enters through gas exchange. Every atmospheric gas is in equilibrium with that gas dissolved in your aquarium. When any gas is in contact with water, some gas will dissolve in the water. The amount that dissolves at a particular temperature depends on the pressure, or partial pressure, of the gas. The dissolved gas and the undissolved gas are in equilibrium.

CO2 can dissolve in water 200 times more easily than oxygen, because carbon dioxide has a slight negative charge near the oxygen and a slight positive charge near the carbon. CO2 is soluble because water molecules are attracted to these polar areas. The bond between carbon and oxygen is not as polar as the bond between hydrogen and oxygen, but it is polar enough that carbon dioxide can easily dissolve in water. The amount of CO2 dissolved in water will always be less than that of CO2  in the atmosphere, say 10 parts per million for your aquarium versus 412 part per million in the atmosphere. But in a low tech tank, the surface agitation will allow a greater amount of CO2 from the atmosphere to dissolve in your aquarium water to replace the CO2 that your plants are consuming.

The plants in your aquarium are either consuming CO2 and giving off O2 in the daytime because of photosynthesis, or conversely at night time consuming O2 and giving off CO2, again because of photosynthesis, which runs in reverse at night. And because CO2 changes the pH of your aquarium, your pH swings up and down on a day/night cycle as seen in this data I collected recently from one of my aquariums:

So, if you are not injecting CO2 in your planted aquarium, surface agitation will help maintain CO2 levels in the water. But paradoxically, if you are injecting CO2 and changing the natural equilibrium then just the opposite will be true and the agitation will cause the CO2 in your water to more quickly reach equilibrium with the atmosphere, thereby lowering the amount of CO2 dissolved in your water.

[Kalita]

Thank you Daniel for your great explanation 🙏 Your time and effort is appreciated by my curious nerd brain that can now rest a bit easier 😁 

[Dirtydave]

Thank you very much.I love to learn .65 and still learning.

[Mmiller2001]

Low tech tanks will have about 3 to 4ppm CO². And these ppm are mostly consistent. The truth, high tech tanks absolutely want excellent surface agitation. High gass exchange is vital in maintaining stable CO² levels. This is an easy read article about CO² and Gass exchange.

https://www.2hraquarist.com/blogs/choosing-co2-why/how-to-push-the-limits-of-co2-safely

Edited March 7, 2021 by Mmiller2001

[kammaroon]

4 hours ago, Mmiller2001 said:

Low tech tanks will have about 3 to 4ppm CO². And these ppm are mostly consistent. The truth, high tech tanks absolutely want excellent surface agitation. High gass exchange is vital in maintaining stable CO² levels. This is an easy read article about CO² and Gass exchange.

https://www.2hraquarist.com/blogs/choosing-co2-why/how-to-push-the-limits-of-co2-safely

Spooky, I was just reading that article before coming onto the forum. It's a really interesting site. There is another article which suggests natural environments supporting abundant plant growth typically have 10-40ppm CO2.

https://www.2hraquarist.com/blogs/choosing-co2-why/why-inject-co2

That would explain why some plants need CO2 injection.

[Kalita]

I read that article too. Loved it!

 

[skipper]

This thread is spectacular! @Kalita I too have a curious nerd brain ☺️  I’m wondering if anyone here could chime in re: if/how the pH shifts could affect the efficacy of meds being dosed? Conversely, would the time the pH is “out of range” not last long enough to induce a clinically significant effect? My box of Paracleanse says ideal range is 6.4 to 7.6. The graph @Daniel kindly posted shows pH outside that range daily. I know ideally we use meds in a likely non-planted QT, so it’s likely not an issue for most. I’m asking out of curiosity. 

[Daniel]

2 minutes ago, skipper said:

This thread is spectacular! @Kalita I too have a curious nerd brain ☺️  I’m wondering if anyone here could chime in re: if/how the pH shifts could affect the efficacy of meds being dosed? Conversely, would the time the pH is “out of range” not last long enough to induce a clinically significant effect? My box of Paracleanse says ideal range is 6.4 to 7.6. The graph @Daniel kindly posted shows pH outside that range daily. I know ideally we use meds in a likely non-planted QT, so it’s likely not an issue for most. I’m asking out of curiosity. 

But if you look at that graph you would see that if you measured you pH everyday at any time in the morning or early afternoon, your pH would always be "in-range". But if you measured pH in late afternoon or evening it would be just the opposite. My guess is most people don't realize how much the parameters swing around in their aquarium based on time of day. Especially planted aquariums.

[skipper]

@Daniel that is exactly why I found this so fascinating. I’m new to the hobby, so I check my water parameters often, mostly to reassure myself. Always after I get home from work, always in the late afternoon.  Never thought I’d say this, but I’m excited to check my parameters tomorrow before sunrise. I don’t expect as dramatic a swing for me because I’m still building my tank and have <10 low light plants. Emersed pothos I don’t suspect would have an effect. But I’m still curious to see if there’s a slight difference.

[PlaneFishGuy]

@Daniel  love the graphs you post. Have you ever looked at how much flow on an airstone affects your tanks? Ive seen information as it relates to water flow in the tank but haven't came across any thing that discusses it from the perspective this thread is getting at. 

I have a never-clog stone tightened for fine bubbles and running just slightly dialed back from wide-open with a USB nano pump in a heavily planted tank. I really didnt know if I should be doing low flow/big bubbles, high flow/small bubble, etc. or if it even matters.