# maximizing Co2 in a planted aquarium question



## atc84 (May 18, 2013)

Currently i have a 1.5 gallon that is serving as a sort of experiment. It has a 13 watt cfl over it, dirted with grainy "dead" looking dirt mixed with a bit of MC potting soil(less than 3/4 inch i'd say), capped with sand. No filtration or critters right now.

How long does the dirt release Co2 into the water column? I assume this is entirely based on the available organics and the decomposition rate of said soil, size of aquarium, etc. But assuming i wanted to maximize the amount of Co2 in this tank (since there is and will be no shortage of algae with this kind of bulb), when would i want to add surface movement to increase gas exchange, which will in turn allow an increase in Co2 (pointed out by D. Walstad recently)?

Thanks!


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## Skizhx (Oct 12, 2010)

I'm curious to know when Walstad began recommending gas exchange at the surface for CO2 (if you'd have a link?). Both her book and some of her forum comments have recommended little to no surface movement.

To the best of my knowledge CO2 is terrible at dissolving into water from the atmosphere. If you have anything producing CO2 in the tank, it's a safe bet that the equilibrium will pretty much always shift towards outgasing from the water rather than taking in more CO2 from the atmosphere.

Could be wrong so I'd like to hear from her recent change of opinion.


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## SBS (Feb 26, 2013)

> I'm curious to know when Walstad began recommending gas exchange at the surface for CO2 (if you'd have a link?). Both her book and some of her forum comments have recommended little to no surface movement.


I am pretty certain she recommended some water movement but not in the book, it's somewhere here in this forum in most recent posts. Obviously you don't want waterfalls and splashing.



> How long does the dirt release Co2 into the water column?


From what I understand, they'll release co2 as long as there's oxygen, organics and nitrogen and best rates are achieved in neutral Ph in the soil and higher temperatures(though this affects the amount of O2 in water so it can be counteractive)

I've expanded a bit below. It's not very scientifically written but I've thrown just my thoughts derived from reading. I can't be certain if that's how it works but that's what I understand without having the scientific methods to test any of it.

Oxygen:

The surface movement recommended has nothing to do with trying to dissolve CO2 from air via gas exchange(though it can happen) but to take O2 from air constantly which is vital for decomposition in the soil, fish and nitrification. O2 is way less soluble in water than CO2 and is actually harder to keep a good level of.

The more O2, the more co2 produced. There are other types of decomposition, which we know by the name of "anaerobic decomposition" and these types don't need O2, but that's what we try to avoid as they produce hydrogen sulphate and methane gas, etc.. and not CO2.

If you have no surface movement at all and you have still water, then the surface of the water will be in equilibrium, but the lower parts of the fish tank will not be because you need some movement to replace the saturated surface water with water from lower levels which aren't in equilibrium with air. This may sound good at first if you are lucky that you have enough co2 and o2 from decomposition and photosynthesis respectively in still water at lower levels of the tank, but not a guarantee if the tank is high in organics. Then lack of O2 can affect the tank, especially in tanks high in organics as between nitrification, fish oxygen uptake and nitrification you may be running out of O2 pretty fast and then who cares about co2 levels if your soil goes anaerobic, you fish can't breathe and nitrification can't go on properly resulting in toxic chemicals lurking in the tank.
Also, in partially oxidized soil the product of decomposition additionally releases some sort of acids(citric acid) and alcohols(ethyl alcohol)
I'd think in a tank as small as 1.5g it doesn't matter much if you have movement or not but in a bigger tank you definitely need some movement. I had 3 external filters with spraybars moving the surface all along my 5f tank and growth and health of plants was great so the old tails about water must be still doesn't apply in practice. I have quite a bit of surface movement in a small 7g tank and it's been doing great. So surface movement has not affected my tanks negatively in practice so I don't dwell on the theory part much and always have surface movement.

Other factors

Soil Ph
Also, there are two other factors besides oxygen that affect the microbial health and decomposition.
Decomposition in acidic soils is way slower than in alkaline soils apparently and best is at neutral soil Ph at 7. So if your water is acidic, adding dolomite to the soil to raise the ph a bit is beneficial. And you obviously don't want the soil to be too alkaline too. I personally have no idea how to control that but I guess since soil becomes more acidic over time you have to ensure it doesn't go too acidic and stays neutral or slightly alkaline but not too much.

Nitrogen
And the other factor I read about is nitrogen. Apparently nitrogen is essential for microbial growth and thus lack of it would affect the efficiency of decomposition. This is obviously not a problem at the start of the tank but it could be in a mature tank. So if you think the tank is running low on nitrogen, you may be affecting not just the plants but your co2 producing bacteria so dosing some isn't out of question even in a low tech tank. This would maintain their health of the bacteria and avoid nitrogen deficiency as they'd need to compete with plants for it.

Temperature
Temperature, the higher the temperature(to an extent) the faster the decomposition but high temperature water holds less oxygen so you must supply way more oxygen in such conditions.
However, different microbial activities have different optimum temperatures.

There could be other factors, but that's what I can't think of right now and have read about.


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## Phoenix1307 (Jun 1, 2014)

In most tanks, probably the fish, etc. contribute the most CO2, with air-exchange being a small percentage. This is probably more of an issue in smaller tanks where the number of fish will be limited in comparison to a larger, deeper tank. Also, I'm not sure if "experimentation" in a 1.5 gal tank equates to the practical balance that is, or can be achieved in a larger tank. Soil works; why not just go big?


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## Skizhx (Oct 12, 2010)

> Soil works; why not just go big?


Big tanks bring a whole other slew of headaches... Cost aside, it's just harder to do things in a larger tank (ie. replanting or starting over again if things don't work out).

Not to mention how impractical it is to adequately plant a larger footprint for these setups.

True that they're more stable. But getting them to that point can be a bit of a commitment.



> I am pretty certain she recommended some water movement but not in the book, it's somewhere here in this forum in most recent posts. Obviously you don't want waterfalls and splashing.


As far as I'm aware, Walstad did recommend some water movement, but she specifically advised against surface agitation. There was a thread on here asking about it that she replied to.


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## atc84 (May 18, 2013)

Are you sure respiration contributes to the most CO2? From what i've read, gas exchange should have a much higher ability to add co2 into the water, rather than the very small organisms inside it. 

Water movement is not necesasry for small tanks, yet surface movement is another thing. Since water movement creates more surface area for gas exchange, more O2 leads to increase decomposition, which in itself ends up with CO2.


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