Plants need Carbon dioxide (CO2) for the process known as photosynthesis by which they make their food. Aquatic plants are no exception, and different species have differing requirements.

To show how CO2 plays a crucial part in photosynthesis, I will show a small diagram describing the process:

Carbon Dioxide (CO2) + Water (H2O) + Light -----------> Glucose (C6H12O6) + Oxygen (O2)

The relationship between Carbon dioxide, light intensity and the rate of photosynthesis...

Photosynthesis also has one other important requirement: Light. Without light, plants would cease to exist and food chains would collapse. The intensity of light directly influences the rate of photosynthesis.

Allow us to imagine a simple experiment, consisting of a desk lamp and jar of water containing a piece of Canadian pondweed (Elodea canadensis).

Looking back at the equation for photosynthesis, we can see that oxygen is one of the products. You would see this oxygen being produced by aquatic plants as small bubbles of gas. If you move the source of light closer to the jar and waited for a minute or two, you would notice an increase in the number of gas bubbles being produced.

However, as the oxygen is being produced, CO2 is being used. Eventually every small trace in the water CO2 will be exhausted and unless it's replenished the plant will eventually die. In Biological terms it is said that Carbon Dioxide (as well as light and nutrients) is a so-called 'limiting factor' of photoynthesis, that is if a factor is in short supply the rate of photosynthesis will be reduced. You may think that the atmopshere surrounding the jar contains enough CO2 to sustain the plant, however the atmopshere actually contains very small concentrations of CO2 and CO2 gas isn't particularly soluble in water at normal atmospheric pressure.

These principles apply directly to aquariums. Before you make the decision to inject CO2, you need to assess the requirements of the plants and the intensity of the lighting over the tank. If you have high levels of light over the aquarium, plants with high CO2 demands and no external source of CO2, your fabulous underwater greenery will start to resemble Shrek's swamp after a while.

In short, if you have lots of light over the aquarium, start dosing CO2 if you aren't already.

What type of CO2 injection should I use?

Good question! CO2 is of course a gas at room temperature, so it would normally come in a pressurized container - like a fire extinguisher. However, there is more than one source of CO2. Depending on your plants' CO2 demands, the intensity of your lighting and the size of your tank, you might not necessarily need a pressurized system.

We will look at the most common sources of CO2 in more detail...

Pressurized Carbon dioxide systems - You can either buy the proper CO2 kits from aquatic stores or if you're game, you can buy a fire extinguisher at wholesale price and modify it to fit a regular and a selenoid valve. The CO2 is actually in a liquid form when pressurized, the actual gas comes from the empty space above the liquid.

Pros - Cheaper in the long run for where you have a big tank or plants with big CO2 demands. No chemicals to mess around with. Containers can be re-filled relatively cheaply.

Cons - Potentially dangerous (see below), initial expense and technical equipment. Make sure you don't buy cylinders with Argon/CO2 mixtures, just pure CO2.

Liquid Carbon additives - Ideal for smaller aquariums. The most popular products include EasyLife EasyCarbo and Seachem Flourish. Liquid carbon additives don't actually contain any CO2, it's just the carbon that's used by the plants.

Pros - Very simple to use, simply follow the label. Low start-up cost. Safe, providing you don't drink it.

Cons - Can get expensive for larger tanks.

Yeast-based CO2 systems - Perfect for the Eco-geek fishkeepers. The simple process of fermenting yeast produces the Carbon dioxide needed. Ready-to-go kits are available or with a little ingenuity a plastic fizz bottle, some CO2-proof hosing (not ordinary airline, CO2 leaks through it!), a check-valve, yeast, water and sugar can be used to create a DIY CO2 system that can be re-filled and re-used again and again. I will write an article on this soon describing the process of making and using a DIY CO2 system.

Pros - Low initial cost, safe. A good way of providing CO2 for undemanding plants.

Cons - Can get messy. The amount of CO2 produced fluctuates constantly. Not particularly reliable or suitable for big tanks or producing large amounts of CO2.

A note on safety involving pressurized CO2...

WARNING! Pressurized CO2 is damned dangerous! If you have any young kids or pets running around the place, make sure you keep any components out of reach of small fingers or paws and fasten the gas cylinder securely to either a wall or inside the aquarium cabinet. A leaking CO2 cylinder will go off like a rocket engine.

How much CO2?

It's important that the amount of CO2 being injected into the aquarium water is controlled. This means the water will need to be periodically tested.

In a normal planted aquarium, the level of CO2 shouldn't exceed 30ppm. There are many ways of testing to see if the CO2 level is too high or too low and whether or not more or less CO2 needs to be injected, depending on the pH and KH (Carbonate hardness) of the aquarium water.

The easiest way of obtaining an accurate reading is to take the pH and KH levels and compare the results with this chart.



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CO2 drop checkers can also be used to guage the concentration of CO2 in the aquarium water. These consist of a glass bubble containing a test-reagent chemical, the most commonly used is Bromothymol blue in a reference solution with a KH of 4 degrees.

The drop checker is kept submerged for 24 hours and colour of the reference solution determines the concentration of CO2. If the solution turns blue, the CO2 level is too low, if it turns green the CO2 level is ideal and if the solution turns yellow (sometimes red) it's too high. The amount of CO2 injected can then be adjusted accordingly.

Commercial CO2 test kits are also available which use the same pH-KH relationship as described above to test the CO2 level.

Dangers to fish from CO2 poisoning and the effect of excess CO2 on pH...

Fish need oxygen to breathe, they are susceptible to CO2 poisoning just like any other living organism that requires oxygen. Excessive CO2 causes a condition known as blood acidosis, which is the primary reason why it can be harmful. How harmful the initial effects of elevated CO2 levels are depends on how quickly the CO2 rises. If the CO2 rises to harmful levels over the space of 1-2 days, most fish can normally adjust by increasing the bicarbonate concentration in the blood and giving you time to counteract any problems.

In water that is high in CO2 and more or less devoid in O2 (Oxygen), fish ventilate more rapidly, become lethargic, flck against objects in the tank and lose equilibrium which sometimes results in death.

However it isn't as straightforward as that, water that is completely devoid of CO2 and high in CO2 can cause fish to develop the exact opposite of acidosis: a condition called alkalosis whereby the concencentration of bicarbonates in the blood is too high. Fish suffering from alkalosis often have skin with a milky appearance, show spread-out fins, rapid breathing and secretions of mucous from the gills.

The only way to treat CO2 poisoning is to cut off the CO2 completely. Aquariums that have very little surface agitation are particularly prone to a build-up of CO2 due to a lack of gaseous exchange at the surface between the water and atmosphere. This can be avoided by increasing surface agitation. A gentle ripple is usually enough to provide enough dissolved oxygen and prevent too much CO2 (which remember, is valuable to plants) from being driven off.

Another effect of CO2 build-up is a fluctuating pH level. Carbon dioxide is mildly acidic and can cause the pH to drop when it builds up due to a lack of surface agitation. A pH that swings constantly (and especially due to a low KH) can be harmful. If necessary, use a pH buffering powder or bicarbonate of soda to increase the KH. It is the carbonate hardness that provides the buffering capacity of the water and prevents the pH from dropping.

Copyright © Chillinator - 2011. This article may not be reproduced without expressed written permission from the author.

Disclaimer: The author accepts no responsbility of any kind should you receive injury or financial loss in the process of pursuing your hobby. If you are unsure as to what you are doing, consult an experienced fishkeeper.

Could one of the Moderators please very kindly stick this thread.

Yep! like now please!

I suppose now is the time to PM a mod and get this thing stuck...