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Chesh 06-06-2012 08:52 AM

Co2, O2, and surface agitation
Should a low-tech tank to have very low surface agitation to allow CO2 to build up in the water and encourage plant growth?

This is a subject that I've been trying to understand - and there seems to be quite a bit of difference in opinion on the matter. . . I thought that gas exchange at the surface was a good thing, as a general rule, but now I'm not so sure!

What is best for the fish AND the plants?

Some think that C02 is the limiting factor in a low-tech planted tank, and since CO2 escapes through the gas exchange created by surface agitation, this should be limited to encourage plant growth. But I've also read that having the gas exchange keeps the levels of CO2 more constant within the tank, and since CO2 levels are naturally lower in a low-tech tank, surface exchange can actually raise the amount of CO2 by pulling it in from the outside air. . .

Then there is the question of the fish, who need the oxygen. I've read that, within a 24 hour period, plant-life in the aquarium will add more oxygen than it uses, but when the lights go off and photosynthesis is halted, the plants continue to respirate - along with the bacteria and fish - and oxygen levels can drop dangerously low in a well planted tank. (not sure what the definition of well or heavily planted is, either!) Because of this, many recommend increasing surface agitation only when the lights are out, so the fish aren't in danger overnight.

But. . . the other side says when the lights are out is when the CO2 raises, and the plants need this in the morning to grow well, if the bubbler is left on overnight they will not get enough to thrive.

Then there is the question of consistency, many feel that whichever you do, it is best to be constant with anything in an aquarium, and keeping the surface agitation on at all times will be better for the fish and bacteria than flipping it off and on. . .

PLEASE can someone help me understand this!!!?

Geomancer 06-06-2012 09:22 AM

I think you'll find differing opinions in just about everything with fish keeping.

I can't say I've ever heard of 'dangerously low oxygen levels' in the morning. You've seen my 125 gallon with the number of plants in it. I would call it well stocked with plants. It is not a total jungle, but more then moderate (in my opinion). The fish are not at the surface gasping in the morning, so I can infer from that they have enough oxygen. That tank has no surface agitation of any kind. The canister spray bar is located below the surface, and does not point up. I think this is the case for almost everyone here who uses a canister.

In my smaller tanks I have HOBs, so they obviously provide some agitation. I have noticed that my 125 has much faster growth than my 20 and 10 gallon (and they are jungles). I can only assume that is because of higher CO2 levels since all the tanks are dosed equally with fertilizer, and have similar lighting levels. But really, every tank is different so *shrug*

If all you have are a couple slow growing plants like Anubias and Java Fern, then perhaps you need to be more careful, but I don't think there is anything to worry about in a moderate or fully planted tank.

Will surface agitation prevent plant growth? No, but maybe a little slower. Maybe not.

Chesh 06-06-2012 09:40 AM

Thanks, Geo! I'm probably just over-thinking things, as usual! I DO have an HOB filter, so there is some surface agitation (tank is 29g tall). I'd still like to understand what everyone is talking about here. What, exactly, is happening in there with the Co2 levels and oxygen - and why. If I had the money I'd buy equipment to test it, just to know!

Termato 06-06-2012 11:03 AM

Additional Thoughts
As far as I am aware of this you are right. You do need surface agitation in order to provide some oxygenation and provide C02 in the water.

You want to see some bubbles when the water goes into the water from the HOB filter outlet. If you don't, you can achieve this by lowering the water level slightly. I personally don't worry about it too much because I fill my tanks up to the top as much as I can and over the week they evaporate creating enough of a space for this to happen.

Water changes are especially important in this as well because when you pour the water you are aerating the water by exposing more surface area in a fast moving current in broken down pieces of water. It traps air and pulls it into the water.

Plants also use Oxygen at night and it is just as important as C02. As far as I have read and have been informed.

A lot of people choose to use air pumps to help with aeration. This is not necessary.

In a well established tank, the built up sediment in the substrate will release more than enough C02 for your plants to grow. This is a mature tank, which neither of us have yet.

That is basically what I have learned and gathered on this subject so far :shock::shock::shock: lol


Chesh - Neither of us have heavily planted aquariums. We have moderately planted tanks at best I would say.

I think you are over thinking this. You have a air pump going in the tank?

I think you are fine? Have you noticed good plant growth? I know your fish are healthy.

Why the concern? Just that super curious feeling of wanting to know everything that is going on and why! 8-) ahhahaha keep me informed!


Also I have no idea about this whole C02 and Oxygen levels rising in the morning or w.e. Never researched or even heard of that.


Byron 06-06-2012 11:12 AM

I have raised this question myself on one of the more "expert" plant forums but in spite of a very lengthy and detail-ridden thread there is still no clear answer.

I am still in the camp that believes that in a natural method planted tank, meaning one that does not add artificial CO2 but relies solely on natural CO2, surface disturbance should be minimal as excessive disturbance will drive out CO2 and there will be less for the plants. And CO2 is already the nutrient most needed and in shortest supply to begin with. Several highly-respected authors hold this view, and so far I have seen no evidence to effectively counter it.

As for the fish: oxygen depletion is very unlikely to occur unless you either have the tank way overstocked, or you are adding CO2 artificially--and in the latter, as those of our members who have high-tech tanks will tell you, this is not likely an issue normally. Any increased water movement such as with airstones may be useful in tanks with added CO2, but not in natural systems.

Aim for a balance and nature will handle it. One can spend hundreds of dollars for testing equipment, but what good is that? If the fish are healthy and free of stress, things must be working. I do know that I have to curtail the tank light to avoid algae increases, which will occur when the plants are being limited by some missing factor, and this in a natural method tank is likely to first be CO2. So at that point, the CO2 is at its lowest. During darkness, fish are far less active, using less oxygen and producing less CO2 themselves. I've never seen even the slightest evidence of oxygen shortages.


Termato 06-06-2012 11:24 AM


Nice post. I agree.



1077 06-06-2012 11:38 AM

I have 80 gallon tank that is heavily stocked, heavily planted, and run sponge filter on timer at night.
With fewer fish,, I am sure this would not be needed but it appears to bring the fish some comfort, and me as well.
Who is to say what is ,or is not heavily stocked, or planted.
Just my two cent's.

P.S. I have tried it with,,and without the aeration at night and have seen no difference at all in plant growth .
Try,Do, then judge for yourself.

Byron 06-06-2012 11:47 AM


Originally Posted by 1077 (Post 1108025)
I have 80 gallon tank that is heavily stocked, heavily planted, and run sponge filter on timer at night.
With fewer fish,, I am sure this would not be needed but it appears to bring the fish some comfort, and me as well.
Who is to say what is ,or is not heavily stocked, or planted.
Just my two cent's.

P.S. I have tried it with,,and without the aeration at night and have seen no difference at all in plant growth .
Try,Do, then judge for yourself.

I have sponge filters in three of my planted tanks full-time and I see no issues, so I would not expect this to be causing problems.

Chesh 06-06-2012 12:54 PM

Thanks for the info, everyone! I do have a HOB filter - so the surface of the water does see steady movement. But the plants seem to be doing okay thus far, so I *guess* it's okay. Time will tell on this one, I guess. It surprises me that there isn't more hard scientific evidence for this type of thing, though I guess it will always vary tank to tank, so maybe it's just impossible to know for SURE. As long as my fish seem fine, and the plants growing - I'm okay with things as they are!

rhymon78 06-06-2012 02:18 PM

I asked this VERY question of Byron a little while back! Great minds must think alike;-)

Byron gave me a very similar answer as he has mentioned in his post here.

I have a filter that sits inside the tank, in the far right corner. It is housed completely in the tank, and the heater etc all together. The outflow (or return) is moveable, and I always had it pointing at the surface to create water agitation in my mind to help oxygenate the water, I do not run an air stone/bubbler.

What made me ask the question, as it wasn't purely based on the effect it would have on the plants, is that my newly acquired floating Salvinia was very small and was being swirled all around and pushed against the intake of the filter. What I wanted to know was, could I turn the filter downwards and into the tank, so that the surface would be still and my floating plants could float nicely. I was concerned that the lack of agitation would hinder the oxygen supply to the fish.

Byron explained as mentioned earlier, and I jumped for joy and turned that pesky return pipe round. Beautiful floating plants ensued, and everything was hunky dory.

I then stupidly added a Kribensis to my community tank that includes a bolivian ram.... MAJOR MISTAKE!!!!!! long story short, after 4 days that Krib went back to the LFS. but.......

The morning I took the fish back, I had a nitrite spike out of nowhere, the fish were gasping, even my loaches, they were all at the surface.. I was going nuts. I got some prime, and did a few major water changes, got rid of the krib, and eventually things got back to normal.. I also moved my pipe back to the surface. I am not saying that the lack of movement had anything to do with the near catastrophe but I wasn't taking any chances, and I have had the return pipe pointed skyward ever since....

You have seen how many plants I have in that 29g, its a veritable JUNGLE... as some members have pointed out:-D and to be honest everything grows, I do dose Flourish twice a week usually, but no CO2, and water agitation constantly. Who knows, what works for one doesn't for another, in my opinion the presence of increased CO2 levels are a matter of worry for those that have high tech aquascaping aquariums, you know the type I mean, award winning underwater scene stuff. Here the fish are a secondary feature to the plants, and are very lightly stocked, so the injection of CO2 and non existent water agitation is fine.. the type of tanks that we, (this includes Termato also) have in my opinion is a non issue. Plants will grow, enough for what we want to achieve.

I would rather have healthy fish, than optimum growth, my tank is too small at it is, too much growth would mean more pruning!

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