The need for Carbon Dioxide (CO2)

Plants need a constant supply of CO2 to thrive - In nature CO2 is continually replenished from groundwater, and from the respiration of organisms breaking down waste matter.
In an aquarium, CO2 is also continually produced, mainly as a waste product from respiration by bacteria and the breakdown of waste material. However, the typical amounts produced fall way short of the needs of well-planted tanks and if we do not add extra CO2, plants find life a little harder and have to expend more energy to grow. It is perfectly possible to grow healthy plants without extra CO2, but giving that extra boost can make all the difference.

Options

There are many products on the market to supply CO2 to your plants, and the best are based on a pressurised canister, regulator, and diffuser. These units are much better than anything which can be home made, but they are expensive at £150 or more, and need monthly refills at £5-£10 a go. If you can afford these, buy them.

A cheaper option is to purchase CO2 generators based on fermentation, which typically cost between £30-£50 and require fortnightly refills at about £3-£5 a go. These generators are actually quite simple to make and unfortunately for the manufacturers, we are going to show you exactly how to make one, and a better one at that!

How it works

Carbon dioxide is a natural by-product of respiration, or energy conversion, and this can be harnessed in the fermentation process commonly associated with brewing. Yeast, when given a source of food, such as sucrose (a form of sugar), a medium (water), and a sealed environment will begin to ferment, converting the sugar into carbon dioxide and alcohol. All that needs to be done to create an aquarium CO2 generator is to harness this process in a sealed container, and force the resulting CO2 into the aquarium water.

The generator

Fermentation produces, in technical terms, lots of gunky stuff, so we need to keep it away from the aquarium. We also need to capture the CO2 in a pressurised container so that it does not simply float away into the air. The simplest object suited to our needs is a 2-3 litre fizzy drinks bottle, these are designed to hold gas under pressure and large enough to contain our ingredients. Once filled, the bottle should be at least a third empty, giving space for gunk to build up without reaching the top of the bottle.
The diagram to the right shows how two generators can be used together for larger tanks.

The mixture

To get the best CO2 producing mixture may take a little bit of experimenting due to variations in water conditions and environmental factors. The standard mix to start from is about a cup of sugar (granulated cane sugar), a half to flat teaspoon of bakers yeast, a teaspoon of baking soda, and 500-1000ml of tepid water. The sugar, yeast, and warm water should be added to the bottle and briefly mixed. Baking soda is added to the mix to help stabilise the solution and also helps to reduce the amount of gunk produced. This mixture should start producing CO2 within 24 hours and will continue for about two weeks before requiring cleaning out and restarting. Both baking soda and active dried yeast are available from any supermarket.

The delivery

Getting the CO2 from the generator (the bottle) to the aquarium is the most important bit to get right - it needs to be completely sealed, robust, and avoid moving gunk into the tank, or water back down the tube. Proper CO2 tubing designed for the more expensive CO2 units is best, but a cheaper alternative is silicon airline, which will work perfectly well for a number of years. Always use the flexible 'rubbery' type, rather than the more solid plastic airline, which will soon degrade with prolonged CO2 contact.

Attaching the airline to the bottle cap is a simple affair of drilling a suitable sized hole and siliconing the airline in place. Silicon is better than glue, which may react with CO2, and often does not attach too well to bottle caps. If you want to do a really professional job, you can use a 'nylon bulkhead fitting' which apparently is used in model remote controlled aircraft as a fuel line and available from specialist model shops.

Blockage prevention

The technical 'gunk' produced from fermentation should remain in the bottle if there is enough air space, but nothing is perfect and it may well enter the tubing where it can block and cause a pressure build up, or enter the aquarium. To avoid this we need a separate chamber where any stray gunk can be separated, and only the gas continues to the aquarium. The simplest way of doing this is to use a 'bubble counter', a simple chamber with a line in and a line out at the top. In this system the gas and gunk enters the bubble counter, drops to the bottom, and only the gas continues out of the chamber. Whilst you can also make this yourself, using a bubble counter (£5-10) is easier and reduces the chance of leaks caused by bad handiwork. Once the airline leaves the chamber it is a good idea to include a non-return valve, which prevents water from the tank siphoning down the tube.

Diffusion

Once our airline producing CO2 reaches the tank we need to diffuse the CO2 into the water. Simply letting the bubbles float to the surface losses virtually all the CO2 to the air, in order for it to diffuse into the water it must have a longer contact time. There are a number of ways of doing this, including creating a 'bell' chamber to hold the gas under water, and this can also be home made with relative ease. In this case a professional CO2 diffuser is the best bet and can be bought for under £10.

Increasing the CO2 output

Using our mix above will produce enough CO2 for about 100 litres. Increasing the size of the mix, or adding more yeast will initially produce more CO2, but the reaction will also last a much shorter period of time. This is because the reaction also produces alcohol, which steadily kills the yeast and stops the process. Instead, if you want more CO2, you can simply plumb two generators together with a simple airline t-piece, and this can be repeated as many times as required. Using two generators also allows you to change one each week, giving a steadier production rate over time.

As mentioned at the start of the article, a home made system like this is nowhere near as good as a professional system, but it is much better than nothing, and is just as good as fermentation based products already on the market. You may need to experiment a little to get the best results, and situating the unit in a warm spot will also help the reaction, but for under £20 including the fizzy drink, tubing, counter, diffuser, non-return valve, sugar and enough yeast and soda for a year, you might as well give it a go!