How come non CO2 tanks and CO2 enriched tanks work?

Tom Barr

Staff member
Jan 23, 2005
Often times someone will post a question on the web, " How do I stop all this algae!!!" They describe it as Black Brush Algae (BBA). Many well meaning folks will suggest adding CO2. These same folks are the "CO2 users". Another, but sometimes rarer group, but no less right or wrong will say stop doing water changes, add some SAE's to chew on it, trim off what is there, reduce the lighting perhaps.

Both methods are right.

But what is the newbie to do?
Often the newbie needs to know which path they are most interested, the eye candy of the CO2 tank, or the ease of maintenance of the non CO2 tank.

Often times I suggest both, this way folks have an understanding of both methods and it's a good excuse to add another tank :D

The SAE's, the reduction of light, trim off any that is there are all things that will help for BOTH methods. The difference is adding more CO2, or not doing frequent water changes(less the better).

So what is going on here?
CO2 stability.
BBA will grow well at 10ppm in a stream. This is a common place for it (or a few of you folk's tanks :rolleyes: .

In the high CO2 tank, doing the water changes added CO2 rich replacement water, since you already have high CO2, this has little impact on the algae.
Algae like CO2 as much as plants, like any nutrient.

But what about the non CO2 tank?
Why does not doing the water change work? This seems counterintuitive? :eek:

What are typical CO2 levels in the non CO2 planted tanks???
(Excel does not count-> see CO2 methods but reduce the growth rates by 1/2 to 3/4ths)
Very low and little. The AM times start out at 4-5ppm, 3 perhaps. Not much variation above ambient after 12-14 hours of darkness and filtration.
They drop in about 1-2 hours to near zero. pH goes up.

Algae like CO2 also recall.
Some algae under higher light have enough energy to drive the reduction of HCO3 to CO2, so do some plants. Often it's wise to have a few bicarb using plants in non CO2 tanks.

If you come along and suddenly dump a bunch of CO2 rich tap water into the tank, what occurs? Which can respond faster, algae? or the plants?
Both will get a boost, but then you are left with the algae which hangs on and starts growing. Next week, you prune off all the algae, then water change, again, the algae keeps coming back.

Allelopathy was suggested, but activated carbon would remove such chemicals so you can see if this truly supressing the algae under good growth plant conditions. This is a standard control method when doing research on allelopathy. The other problem with this is that good plant growth, regardless of species, produces the same effect when the relative plant biomass is fairly high. So the effect would require that all plants illict the same type of allelopathy intensity from some 300 plant species. The odds of this occurring are virtually improbable.

Fe limitation was suggested: simply add it and see the results(I've done this for sometime without issue). Plants leak Fe anyhow as well as most any nutrient.

PO4/NO3 excess, well, again, add some to an other wise stable tank, see if you can perturb the system and get an algae bloom.

I have not really pushed this one, but I have gone to 2 ppm PO4 and about 30ppm NO3.

Substrate fertilization can reduce the need for most dosing by weeks, months even due to the slower growth rates. Less light ( a point) will also reduce the nutrient demand from a tank and affords more wiggle room in supplying nutrients to any method. More light is not better, there is a strong tendency for new folks to get high powered light and go cheap and DIY on CO2, this causes many heachaches. Likewise for the non CO2 approach, many newbies add lots of light or had someone tell them they do not need CO2(at face value this is true BTW!!).

Water column and substrate fertilization is often a contentious issue. The reality is that all tanks experience both.
CO2 is taken up foliarly in the water column, there are no aquatic plants that get all their CO2 from the substrate(Lobelia d. is a good one for this but these are the exception), Ca, Mg, NO3 etc.

The sand in our tanks is porous and nutrients freely leak out of the substrate. Caps of inert materials will hold the nutrients in place for a while but only for so long. Plants also leak nutrients, roughly 10% of the fixed carbon are lost. This ends up in the gravel and the water column.

The real question between these two different methods is what trade offs does each method have?

In the past, many substrate folks suggested that PO4, NO3, Fe excesses in the water column caused algae, today in many circles that have looked at this in more depth, this is clearly not the case.

So this is not an advantage.

General trade offs:

Non CO2:
1. Slower growth(this can be bad, or good depending on the goals of the aquarist)
2. No water changes, this can only be a positive! Well, maybe 1 out of 100 thousand folks might miss their weekly water changes etc?
3. Herbivores do more work per critter! Slower plant growth= slower algae growth.
4. Slower response times, allows the user to neglect the tank longer.
5. Excel can be used to address more growth needs, algae issues.
6. Additional nutrients can be dosed(see non CO2 method article here) to produce better growth and a wider selection of plant species.
7. Substrate can supply the nutrient demands easier(no dosing every few days etc) and longer times frames
8. On any given day, unless a lot of work is done on the CO2 tank consistently, these non CO2 after a peroid of grow in, often look nicer.
9. This one kills folks: less algae.

There are other trade offs not mentioned but these are just a few......


1. Faster growth
2. Can keep generally a wider array of plant species
3. Lots fo pearling
4. Water changes are an effective tool to control nutrient levels, remove waste, mulm/detrital material, algae
5. Substrate disturbances are more ferequent=> need to do water changes anytime a significant replanting is done(same is true, but far less frequently done in a non CO2 tank)
6. Brighter lights can be used, but..........certainly are not required
7. Uptake rates can vary from 5-15 x faster than non CO2, this requires dosing of other downstream nutrients such as NO3, K+, PO4, Fe etc
8. Lots of work pruning , fiddling with CO2 levels, dosing, more work in general.

There are many other trade offs for each, but over all, many folks come to the idea of havign a planted tank and do not want a fast hard driven tank, but many are easily swayed with eye candy and CO2.

If you tell them no water changes, the fish waste => plant nutrients, a good substrate, less pruning, less algae, fully planted, most will go for it.
Later, they can step up to CO2 if they desire(most will at some point).

Main thing is to have a high success rate and meet the demands and expectations of the newbies.

This means helping them and having a high success rate when you do help them.

The CO2 trade off: more plant species and lots of pearling. Nice eye candy examples that they can try, more things to play with such as test kits, CO2 equipment, many enjoy learning what it is all about. Algae bouts are often worse, genenrally====> you guess it 90% CO2 related issues.

So not using CO2 is not a bad idea for many folks and how many have not ever negelected a much needed water change?

Many want to garden and are excited about the speed the plants grow with high light, CO2 and lots of nutrients.

That certainly hooked me on plants and I likely would never have gotten into them today at this level if I'd never used it, but I've always like the low key layback patience approach as well.

There are strong arguements for both, but the advice needs to address what the newbie is really after, that is an often difficult ambiguous question for them.

Success with plants is the main focus and having some thign that will look nice over the long term. Knowing both methods will better help you all to help newbies and not sway them inadvertantly one way or the other.

They need to decide of their own free will which method is right for them. Later, they can try the other method if they chose.

If you seem to fail with CO2, non CO2 is an option. If you have trouble keeping your hands out of the tank or really want the Amano like scapes, you can try the CO2.

There are potential scapes that can revial CO2 tanks, but few folks try these methods in the last 15 years. See some older Dutch scapes for the 1940-1970's that never used CO2.

Knowing what we know today about higher faster growth CO2 enriched tank, we have a good understanding of nutrient dynamics and plant deficiencies and can scale these down to the slower growth rates associated with non CO2. This will help increase the plant health and success rates of the non CO2 planted tank as well as come to terms with the differences in these methods which often appear at first glance to contradict one other.

Knowing the trade offs and methods wil help you help folks better and have a wider range of skills and help address the real issues with all the differences on the web. Plants all grow for the same main reasons, same with algae, but the rate at which they grow can allow many methods to be used.

Enjoy and brach out and try both methods if you are only doing one.
Non CO2 tanks are easy to set and maintain and are cheaper.
CO2 tanks can be done at a small scale also, a good beginner article is Dan's old article here:

If you like either, you can scale up and get a good feel for it before taking the plunge! Then you know what method you enjoy more anmd is right for you and your goals as well as when you help other people.


Tom Barr