Frequently asked questions on adding carbon dioxide

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Matt Clarke answers some of the most frequently asked questions on adding carbon dioxide to your planted tank...

> What is carbon dioxide?

Carbon dioxide (CO2) is a gas formed from a carbon and two oxygen atoms. It is the most common gas in water, being found at much higher levels than either oxygen or nitrogen (70:2:1). When it mixes with water some of it forms carbonic acid which reduces the pH.

> Where does natural carbon dioxide come from?

CO2 is produced by fish and bacteria as they breathe. Plants and algae remove carbon dioxide from the water during the day and release oxygen, but at night when photosynthesis stops, they respire, consuming oxygen and releasing carbon dioxide.

> Why add extra CO2?

Adding extra carbon dioxide can dramatically improve plant growth. It can also be used to reduce the pH, which can be beneficial if you keep fish from soft, acidic water regions, like the Amazon basin.

CO2 can be added to specialist planted tanks using a special carbon dioxide dosing unit. Proper pressurised dosing units start in price at around 100, but you can pick up a very basic yeast-fed model for about 20.

> How does a carbon dioxide unit work?

Gas is released from a pressurised canister via a regulator valve and some special plastic hose. The gas is bubbled very slowly through a diffuser or bubble counter inside the tank, which gently releases the gas into the water, increasing the dissolved carbon dioxide level and reducing the pH.

There are also a number of fermentation-based carbon dioxide dosing systems on the market. These are cheap and very basic, but the output of CO2 isn't controllable and the gas production can't be switched off at night, which could theoretically cause problems for your fish.

> Why do I need to switch off at night?

At night plants don't use CO2, they produce it. If you add too much at night the pH will drop causing fluctuations that may be harmful to fish. The levels of CO2 in the water could also reach toxic levels, resulting in a tank of suffocating fish by morning.

Most experts recommend investing in a regulator with a night shut-off valve to turn the gas off overnight. Amano turns off the CO2 supply at night using a special night shut-off valve. You can pick one up for around 40-50.

The use of uncontrollable forms of CO2 dosing, such as fermentation-based CO2 reactors could be potentially dangerous, especially if your tank is heavily stocked with fish or plants. Many fishkeepers do use these products without problems, however.

> I've heard that I won't be able to add as many fish if I add extra CO2. Is that true?

In a nutshell, yes. More fish means there's a greater demand for oxygen. If the tank is a typical planted system, with no aeration and slow filtration, the extra carbon dioxide produced by the additional fish and their wastes can lead to problems in heavily-stocked tanks.

Keep fewer fish and pick the species carefully. As a general rule, fish from slow-moving soft waters are more likely to be able to cope with higher CO2 levels than fish from fast-flowing waters. The more CO2 you add, the riskier things get for your fish.

> Why do recommendations for filtration on planted aquaria differ?

Aeration and water turbulence from fast filtration and water circulation drives CO2 from solution and oxidises plant nutrients. Plant growers call this "over-filtration". (In fact, this is a misnomer, since you can't really overfilter a tank).

To overcome the problems of nutrient oxidation and loss of CO2, plant growers opt for filters with slow turnovers, but a large enough media volume to support the fish. Don't simply use a smaller filter because it has a lower turnover...

> How does carbon dioxide affect the fish?

The ability of haemoglobin to carry oxygen is inhibited under high carbon dioxide levels (hypercarbia). This can lead to suffocation, gasping, general lowered immunity, or nephrocalcinosis.

Different fish have different tolerances to CO2. Some species are extremely sensitive while others don't seem affected at all.

> What if I add too much?

If you add too much the fish will start to gasp, and you'll also play havoc with the water chemistry. Turn off the gas and increase the aeration by adding an airstone, or by increasing the water turbulence by directing filter outlets upwards.

> At what levels does CO2 become toxic?

Adding too much CO2 causes the pH to drop, and it is also toxic to fish at high levels. There is some debate over what represents a safe level. Research from fish biologists and vets suggests that levels over 10 ppm can be dangerous in certain cases.

However, some CO2 system suppliers claim that levels of over 100 ppm are safe for neons, 500 ppm levels are safe for mollies and 800 ppm levels are safe for guppies. In my opinion, it's likely to depend on the chemistry of the water and the fish species. Be careful.

Interestingly, at high CO2 levels and low pH levels, the oxygen carrying capacity of fish blood is dramatically reduced due to the Bohr effect. This means that losses can occur even if the oxygen level is above the normal safe limits!

> How much should I add?

The exact amount required depends on the KH, pH and current CO2 levels. As pH declines, CO2 levels increase. Many plant experts recommend going for a level of 10-25ppm. Dennerle recommend levels of 35-40 ppm. Takashi Amano runs many of his tanks at somewhere around the 15-25 ppm mark.

However, you must remember that carbon dioxide alters water chemistry, and too much can harm the fish. So if you're adding CO2, you should also test the water.

> How can I tell how much CO2 is in my water?

There are two ways of doing this. The first involves using a special carbon dioxide test kit. Calculating the exact carbon dioxide level calls for very accurate CO2 test kits - unfortunately, some aquarium kits may not be precise enough to do this properly and the level of gas in the sample water can drop quickly between removal and testing.

The other way of estimating the CO2 level is to do so mathematically on the basis of the results of a KH and pH test undertaken with a conventional test kit. The mathematical method determines the probable CO2 level based on the pH and KH. Obviously, this method depends on getting accurate KH and pH results from your kit. As previous reviews in PFK have shown, the accuracy of aquarium test kits leaves a lot to be desired, so this method too can give false results.

> Is there a calculator for determining the carbon dioxide level on the Practical Fishkeeping website?

Yes, we've produced a Carbon Dioxide Calculator which works out how much CO2 is likely to be present in your water depending on the results of your KH and pH tests.

This was inspired by aquatic plant expert Chuck Gadd's original program. Chuck's site includes a wealth of excellent technical information for those interested in the safe use of carbon dioxide in the planted aquarium.

>I have used the calculator and it appears that I can simply get the CO2 level I want, around 20ppm, simply by altering the KH. Is that right?

No. You can only adjust the CO2 level by adding more or less carbon dioxide to the tank. You cannot adjust it by meddling with the KH, so adding bicarbonate of soda isn't going to help you. When you increase the KH, you'll also push up the pH.

> I have lots of phosphate in my water. Does this matter?

Yes. The presence of phosphate in the water, either because it's been added by your water authority to protect the pipes, or because you've used a phosphate-based pH buffer (or have a dirty tank) will mess up the KH-pH-CO2 relationship. This could mean that both the test kit method and the mathematical or computational methods of determing the CO2 level are less likely to work. Such methods only work properly in carbonate buffered water.

> What is a long-term CO2 indicator for?

A long-term CO2 indicator is a small clear plastic container which contains a reagent that changes colour according to the pH and KH of the water. It is attached to the inside glass of the aquarium using a small sucker.

Although arguably less precise than a set of test kits, it provides a visual indication to warn you if you're adding too much (or too little) CO2.

This article was first published in the November 2002 issue of Practical Fishkeeping.