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Carbon Dioxide in Planted Freshwater Aquaria
By Karen Randall at Aquatic-Gardeners.org

As with all forms of life on earth, carbon is the substance that is needed in the largest quantities. In fact, carbon accounts for about 43 percent of the dry weight of all plants. Plants have two different ways of finding carbon to use for growth. The first is from free carbon dioxide (CO2) captured from the water, substrate or directly from the atmosphere. In the absence of CO2, many plants have the ability to meet their carbon needs by splitting carbon directly from carbonates in the water, which is the second method.

While this second method is a useful adaptation in the wild, it is undesirable in the aquarium. When plants draw on the calcium carbonate in the water to meet their carbon needs (this process is sometimes called biogenic decalcification), the water has less and less buffering capacity, leading to serious pH fluctuations. Eventually, the plants can totally exhaust this carbon source as well.

There are several sources of CO2 within the aquarium. One fifth of the CO2 assimilated by the plants is actually returned to the water. The fish also add to the CO2 levels with their respiration. The bacterial decay of both dead plant material and driftwood decorations in the tank produce CO2. But the largest contributor to CO2 levels in the aquarium is the biofilter. In fact, a fully cycled biofilter will produce 1 gram of CO2 within 24 hours for every gram of dry food added to the tank.

In a properly stocked, moderately lit (no more than about 2 watts per gallon) tank with minimal aeration during the daylight hours, it is possible to have reasonably good growth without the addition of supplemental CO2. At higher light levels, CO2 supplementation becomes increasingly necessary. If measurements of the pH in the morning before the lights come on, and again in the late afternoon, are drastically different, it is a signal that the plants are using up all available CO2 in the tank. If the alkalinity (or carbonate hardness) of the tank drops over time, CO2 deprivation is almost a certainty.

The amount of CO2 needed in the aquarium for good plant growth is considered to be around 15 milligrams per liter (mg/L), although a range of between 10 to 40 mg/L is considered safe and effective. With good lighting, a moderately planted aquarium will consume 1 to 2 mg/L per during the photoperiod. Room air contains 1 to 3 mg/L of CO2, and aerated water dissolves only 0.7 the CO2 content of the air. From these figures it is easy to see why aeration should be kept to a minimum in the planted tank, and why CO2 supplementation is often necessary.

Without the plants that cover our earth, both terrestrial and aquatic, the atmosphere would not contain enough oxygen for us to breathe. We can use the oxygen-producing capabilities of plants to create a healthier environment for our fish.

Photosynthesis is at the very heart of the food chain. Plants (and certain bacteria) have the unique ability to directly harness the energy of the sun. This energy is used to convert water, CO2 and minerals in the environment into organic material in the form of plant growth. A by-product of this process is gaseous oxygen.

The process of photosynthesis is quite complex, and beyond the scope of this article. Those who are interested, (and it is a very interesting process) can consult any good encyclopedia. For our purposes, the important part to understand is that for optimal growth (and optimal oxygen production), plants must have a light source that is of sufficient intensity, correct spectrum and appropriate duration. They must have access to a continuous supply of CO2 during the hours that the tank is lit, and they must have a reliable supply of all the other nutrients they need.

Novice aquatic gardeners are often concerned that adding CO2 to their tanks will limit the amount of oxygen available for their fish. At the levels we are interested in maintaining in the aquarium, oxygen and CO2 coexist successfully. Adding small amounts of CO2 does not displace oxygen. In fact, the proper use of supplemental CO2 with appropriate lighting and trace element supplementation should actually increase the levels of dissolved oxygen in the water. Often, in a tank with really good plant growth, you will see bubbles rising off the plant leaves. This is oxygen being produced at such a rate that it can no longer be absorbed by the water. Its only course is to escape into the atmosphere.

At the temperature I typically maintain in my planted tanks, around 75 degrees Fahrenheit (24 degrees Celsius), the saturation level for oxygen is just over 8 mg/L (or 8 parts per million). For healthy fish and plants, it is recommended that oxygen levels not be allowed to drop below 60 percent of saturation, or in this case, about 5 mg/L. Below 2 mg/L, or 25 percent of saturation, your fish would be in serious trouble. (These figures are for freshwater only. Seawater has a lower saturation point.)

In my tanks, which are maintained at approximately 23 mg/L of CO2, the oxygen level hovers around 11 mg/L during daylight hours, well above 100 percent saturation. The buildup of oxygen during the daylight hours is such that even in early morning, before the lights are turned on, the oxygen has only dropped to about 8 mg/L. Oxygen bubbles start rising off the plants within an hour or two after the lights are turned on, as levels again climb above 100 percent saturation.

Properly monitored, supplemental CO2 will have no adverse affects on fish. CO2 is a gas that is easily driven off by surface agitation. In a tank with a heavier fish population, if the oxygen level drops below acceptable levels at night when the plants are not photosynthesizing, the operation of an airstone (on a timer set for when the lights are off) can easily drive off any excess CO2 and supplement the dissolved oxygen within the tank. In a properly buffered tank that is not overstocked, however, most people find that the oxygen level in the tank remains high enough, and the pH in the tank remains more stable, if the CO2 is left to run day and night without the interference of nighttime aeration.

Keeping the balance

Plants come from a number of different ecological niches. Some require extremely high light levels and correspondingly large amounts of CO2 and minerals. Many of our more difficult “bunch plants” are in this category. These plants are typically high oxygenators, as well.

Other plants have adapted to areas where the sun lovers can’t compete. These plants are generally slow growers, and do not give off large amounts of oxygen. Examples of these types of plants would be most Cryptocoryne and Anubias species. Many of these plants do well in relatively dimly lit tanks, and are not likely to exhaust the CO2 provided by the fish and biological activity in the aquarium. These plants show little, if any, direct improvement with light and CO2 increased beyond moderate levels.

A third group of plants, which contains the majority of popular aquarium favorites, is more adaptable. While they are capable of doing quite well without supplemental CO2 if given proper care, their growth is truly astounding in an environment of high light, additional CO2 and adequate trace element supplementation. Echinodorus and Hygrophila species come immediately to mind in this category, but there are many more.

It is important to remember that whether you use supplemental CO2 or not, the amount of light, CO2 and available minerals must be in equilibrium. If lighting is increased without adjusting the other factors, there is the possibility of biogenic decalcification. Excess CO2 is, at best, useless to the plants, and, as will be explained below, can be harmful to your fish. If the two major nutrients in the tank (nitrogen products and phosphate) are increased beyond the needs of the plants, you will experience algae problems. Many trace minerals, while absolutely essential to plant growth, are toxic in larger quantities.

CO2 and pH

There is a danger in using improperly monitored CO2. The presence of CO2 in water causes the pH to fall. This can be used to the aquarist’s advantage to lower the pH to the range preferred by most plants. However, if the water is not properly buffered, or if too much CO2 is introduced, the pH in the tank can drop quickly to levels that are lethal to fish. It is important to think of supplemental CO2 as an “additive,” just like fertilizers, pH adjustors, medications or even fish foods — all of which can create lethal conditions in your tank when added in excess. CO2 is no different. It must be used within specific parameters to ensure the safety of tank’s inhabitants.

CO2 produces a weak acid, and, as noted above, will lower the pH of a tank. For people with moderately hard water, this works out perfectly. Aquarists with water that has too much alkalinity may need to reduce the carbonate hardness (KH) by mixing their tap water with distilled or reverse osmosis water.

Those aquarists with very soft water must increase the alkalinity of their water. Without the buffering properties of KH, CO2 supplementation can cause the pH to plummet, to the detriment of plants and, more especially, the fish.

KH can be increased by the use of preparations sold exclusively for this purpose. For the more budget conscious, sodium bicarbonate (baking soda) or calcium carbonate can be added to tap water in small amounts to achieve a KH reading of between 3 and 6. Baking soda will raise only the KH. If both the KH and general hardness (GH) are too low, calcium carbonate is the better choice. One teaspoon of baking soda will increase the KH of 50 liters of water by approximately 40. Two teaspoons of calcium carbonate will increase both the KH and GH in the same amount of water by about 40. Another method of increasing both carbonate and general hardness is to prefilter water through crushed shell grit, crushed coral or dolomite gravel until the desired level is attained.

How much CO2 is enough?

As stated above, the recommended range for CO2 is between 10 and 40 mg/L. The goal should be to find a rate of CO2 supplementation that falls within this range, and produces a pH reading that is neutral (7.0) or slightly below. Avoid the extreme ends of this range. If your delivery system is not absolutely accurate, it is too easy for CO2 to move outside the recommended range.

There are commercial test kits available to tell you if CO2 is within the desired range. It is also possible to estimate the CO2 levels in your tank using the CO2 chart available at the Krib. You must know the pH of your tank water and the KH. Please be aware that this reading differs from total hardness, and buy a test kit that specifically measures the former.

Next month we will discuss the different options for providing supplemental CO2 in the planted tank.

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