CO2 Overview
By: Holly Harrington


To CO2 or not to CO2, that is the question. Well, not necessarily THE question, but it’s definitely ONE of the big questions if you have or are considering setting up a planted tank. What follows is a brief overview of CO2 options. For more details, contact one of the many planted tank gurus on this forum or search out the many discussions on the subject on the Web.

So why CO2 in an aquarium?

Plants require both light and CO2 to photosynthesize and be alive and healthy. That’s true both in your aquarium and on your window sill. A low-light tank doesn’t create a lot of demand for CO2 because the plants are growing slowly. In such tanks, CO2 augmentation is probably unnecessary. But once you add more light (2 WPG is considered moderate; 3 or more high) CO2 augmentation needs serious consideration both for the health of the plants and to prevent opportunistic algae.

There are basically three ways to augment CO2 in your tank. (Your fish are breathing breathing out CO2, but it’s not enough with higher light or more demanding plants). You can add liquid Seachem’s Excel or can use a yeast-generated system, either do-it-yourself (DIY) or store-bought DIY. Or you can get wild and crazy and get a pressurized CO2 injection system.

First, the easiest avenue. Seachem’s Excel is a liquid that provides “bioavailable organic carbon” to your plants. It is readily available online or at your LFS, and is not expensive. According to dosing instructions, it should be administered daily or every other day. How does it work? Well, that’s beyond me, but it does a good job in smaller tanks and those with relatively low light. It does not affect pH.

I started with this option, but as soon as I added plants beyond anubias and Java fern and got a light with 3 WPG, I needed to research other options. (Even with pressurized CO2 injection, I still use Excel in both my planted tanks. Excel appears to have some algae-fighting properties, especially against black beard algae. This is mostly anecdotal and the company makes no claims of this, but in my experience, it is true.)

*** Please see comments below

The second option is the DIY yeast-fermentation option. By combining yeast, sugar and water in a confined space, like a plastic bottle, you produce CO2 that can be directed into your aquarium. DIY systems, the designs for which can be found throughout the Internet, are relatively easy and inexpensive. You can also buy a store-bought DIY system, which I did. I used the Red Sea Bio-Turbo. (There are several other types of systems along these lines.)

The Bio Turbo system comes with the sugar/yeast mixture and a bottle to put it in, a small pump/diffuser for inside the tank and rubber tubing to connect it all. I found it extremely easy to use and it generally worked well. The drawback is inconsistency. As time goes on, the yeast/sugar mixture gets exhausted and the CO2 going into the tank diminishes. You must replenish the solution about once a month. It then takes 24 hours or so, once remixed, to get CO2 going again. I also found that during the winter, when the house was cooler, the CO2 generation slowed below what was acceptable. This type of approach works well for smaller tanks, tanks without too much of a CO2 demand, and as an introductory step into CO2 injection.

The third option is to get a pressurized CO2 injection system. I found this a bit daunting, but it was not as difficult as I’d feared. I purchased a 5 lb CO2 bottle locally and a regulator with bubble counter online for less than $200 total. I used the diffuser from the Bio-Turbo systems, although there are several ways you can diffuse the bubbles. With some assistance, I was able to set up the system in less than 20 minutes.

Using the bubble counter, I set my system on a bit less than 2 bubbles per second. The regulator is attached to the same timer that the lights are, so the CO2 is not running without the lights. Some systems are more complex and come with pH sensors. There are also options using paintball canisters, but I can’t speak to the pros or cons of either systems. I am happy with this option as I like the ease and consistency. I expect the bottle to last me for 6-12 months.

It’s important to note that too much CO2 is problematic, and the levels should be monitored. There is a constant relationship between Kh and pH, which can give you a good sense of your CO2 levels. You are generally aiming at 15 ppm, with a range of 10-20 as acceptable. Anything less isn’t doing any good and anything more can be doing harm. For example, a KH of 3 and a pH of 7 gives you a not-great 9 ppm, while a KH of 5 and a pH of 7 gives you a desirable 15 ppm of CO2. There are charts outlining these ratios elsewhere online. Also, CO2 injection can/will lower your pH, so it’s important that you monitor it closely and make sure your tank is properly buffered.

In short, if you’re at all interested in plants – and for the sheer beauty of them in your tank you should be – you must do your research into the CO2 options. Then it’s up to you to choose the one that’s right for you, your wallet, your tank and your fish.


From: RTR
Overall this is a valuable note and a good introduction to the topic, but I can’t resist adding some footnotes and suggesting some minor modifications:First off, many folks do question the need for CO2, while they tend not to question the need for mineral supplements. One simple fact from analysis of what makes up plants can help with that question. Plants tend to run about 40% carbon, and all the other minerals trail down from there. Plants get carbon from CO2 primarily, almost all atmospheric CO2. A very few aquatics (Val is one) can, under severe need, go to biogenic decalcification and split carbon out of bicarbonate/carbonate. I promise you that you do not want this process happening in your tanks. A pH of about 10 is the common result in limited water volume – as in tanks. So carbon is not just a nutrient, it is the nutrient required in the largest quantity.

Seachem’s Excel is wonderful stuff, and I use it routinely myself, but a couple of cautions are needed. Some fish are sensitive to Excel, especially at higher-than-suggested dosages, such as is needed for algae control effects. Some plants seem to be sensitive in much the same fashion. Use as suggested by the maufacturer initially, but watch the tank closely for a bit. If you want to exceed suggested uses or doses, watch very carefully and be prepared to do serial large-volume partials back-to-back to dilute the stuff out or you might kill your fish. Yes, I have, and know several others who have as well, including some internationally known planted tank folks – it doesn’t just happen to novices.

The inconsistency of DIY CO2 is very well presented, and that is to me the greatest issue with DIY fermentations. There are several approaches which can help that problem though, if they are applicable to your particular set-up, and do not make the chore-load too great for you (in honesty, they do for me). The simplest option is to use multiple bottles, staggering the renewal of each, such that two or three small bottles replace one larger one. You can have much more even production in this setup as the various bottles will wax and wane at different times, the sum being more even. The recipe proportions may be juggled as well, to drive the process faster or slower. To combat the issue of lower ambient temperatures in winter than summer, use a tank with a heater or just larger buckets with heaters. It will steady the production quite well, especially in a multi-bottle setup.

On the use of pressurized CO2, some note is needed that bubble size varies tremendously between equipment types. Bubble counting is great for your own monitoring of gas output and for control of resulting water concentrations, but is worthless for discussion on the forums. The resulting water column concentrations are needed/required for discussions with others.

I also question the concentrations suggested. 15ppm is not sufficient for a high-light full-supplement rapid growth tank. 30ppm is a better standard IMHO & IME – that also is about a full log of pH depression, so is highly communicable level in discussions. I believe you will find that is the standard these days.


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