Tuesday, August 13, 2013

Oxygen and Carbon Dioxide in the Aquarium

Water Quality: Carbon Dioxide

Carbon dioxide is an essential component for good plant growth. It is an important requirement for photosynthesis to occur. The amount of CO2 which is ideal for the growth of plants inside the tank is around 15 mg/L. A 10-40 mg/L range is considered safe and effective. A well-lighted and moderately planted tank will consume approximately 1-2 mg/L during the photoperiod.

How much CO2 is enough?

The recommended range for carbon dioxide in an aquarium tank is between 10-40 mg/L. As much as possible, you should stay within this range while maintaining a neutral pH (7.0) or slightly below.

The need to monitor CO2 levels in the tank is very important for levels can easily move outside the recommended range as a consequence of changes in other water parameters. The Fluval CO2 Indicator Kit can help you monitor and gauge CO2 levels in your tank while the API Freshwater Master Multi-Test Kit is a 5-in-1 solution of monitoring pH, ammonia, nitrite, GH and KH.

In most aquariums, particularly in a planted aquarium, the presence of CO2 does not really pose a threat to the tank inhabitants. However, extremely high levels can interfere in the ability of the fish’s body to efficiently transport oxygen to the tissues. Even if oxygen levels are high, carbon dioxide concentrations that exceed recommended levels can use some fish species to suffocate. If your tank has only a few plants or without plants, a good aerator can easily remove excess carbon dioxide.

In an aquarium that is properly stocked and moderately lit, good plant growth is possible even without supplemental carbon dioxide being added. This is particularly true when there is minimal aeration during day time. With higher levels of lighting, the need for carbon dioxide supplementation becomes progressively more necessary. You will know that plants are using up all the carbon dioxide which is available in the tank when the water pH before lights come on in the morning is drastically different with the water pH in the late afternoon.

Ways to reduce CO2 in the tank

With a good tank aeration system, the buildup of excess CO2 is not really a problem however weekly tests can be undertaken for tanks which are heavily planted to make sure that there are sufficient concentrations of CO2 present for photosynthesis.

1. Surface Agitation – since CO2 is a gas, it can easily be driven off by agitating the water surface.

2. Airstone – an airstone should be set to turn on automatically when the tank lights are off can help eliminate excess CO2 and supplement dissolved oxygen in the tank.

If your source of water is supersaturated with carbon dioxide, excess CO2 can be removed by degassing which is accomplished by vigorously aerating the water using an air pump and airstone for 24 hours before adding the water to your tank.

Carbon Dioxide Supplementation

With regards to aeration, only 0.7 mg/L of carbon dioxide from the air is dissolved in aerated water. This is the reason why a planted tank needs carbon dioxide supplementation with minimal aeration.

If you are thinking of giving supplemental CO2, think of it as an additive just like fish food, fertilizers, medications—use in moderation.

There are aquarists who think that by adding carbon dioxide to their tanks, they will be limiting the amount of oxygen available for the fish population. When you add small amounts of carbon dioxide, it does not necessarily mean you are displacing oxygen. In fact, proper utilization of supplemental carbon dioxide can actually increase the amount of oxygen which is dissolved in the water.

In tanks with good plant growth, you may see bubbles from the plant leaves. This is an indication of a higher oxygen production which can no longer be absorbed thus escaping into the atmosphere.

If you choose to give supplemental CO2, make sure to properly monitor the tank conditions to ensure that there will be no adverse effects particularly on the fish population.

Relationship between CO2 and pH

The need to make sure that CO2 is maintained within specific parameters is very important to ensure the health and welfare of your tank’s inhabitants. When carbon dioxide levels in the water increases, there is a consequent decrease in the pH of the aquarium tank water since C02 produces a weak acid.

In a way, this is advantageous to plants that prefer a lower pH. However, if the tank water is not properly buffered or when excess CO2 is introduced, the water pH can quickly drop to levels which can be dangerous to the fish population. The buffering property of KH protects the water pH from plummeting.

Carbon dioxide deprivation is also a certainty when there is a drop of carbonate hardness and alkalinity over time. In cases where C02 levels in a planted tank is below the normal range, plants will not be able to undergo photosynthesis efficiently thus resulting in poor growth.


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