Everyone knows that every green plant requires carbon dioxide (CO2). Without it, they could not assimilate the required nutrients. Plants get CO2 from the air and from water. The CO2 in water is mainly a product of bacteria breaking down organic matter. In the aquarium, the amount of CO2 that fishes emit through respiration is limited and is quickly used up by the plants. This causes the water to become alkaline or have a high pH value. Providing enough CO2 to the ecosystem of the aquarium is a the most important task for a planted tank keeper. The challenge is maintaining a natural balance between the requirements of the fish and the plants. (Left picture : Microcopy of aquatic plants during photosynthesis)
Various devices are available now available for injecting CO2 to planted aquaria, like the CO2 injection device on the lower right photo below and the CO2 bottle canister (below left photo). If the aquarium is a large one, a computerized pH controller is essential for avoiding chronic CO2 shortages. Smaller aquaria don't need as much control. If the plants aren't growing, they either need more CO2 or more light. Simple, isn't it?
There is a simple way to tell if the plants are photosynthesizing or not. Observe the plants one or two hours after injecting the CO2 into your aquarium. There should be small bubbles forming on the leaves (see left picture), this is a sign that photosynthesis is taking place. If not, and the amount of CO2 has been increased, then the problem most likely has to do with the lighting or the filter.
The level of CO2 in the water can be determined by measuring the pH level. If the pH level falls below the neutral level of 7, then there is too much CO2 in the water, and the water is acidic. If it rises above 7, then there is much too little CO2, and the water is alkaline.
The computerized pH controller operates on this principle, but it is important to remember that it is just a convenient relationship. Thinking that pH and CO2 are the same thing could lead to trouble.
For example, if there is a large quantity of bacteria due to an old sand substrate or old filter material, the pH level will fall. This is caused by either the bacteria's respiration or the nitrate byproducts of its decomposition of ammonia. If the fall in pH is caused by bacterial respiration, it is good to increase the amount of CO2. But if it is caused by an increase in nitrates, it is better to add more water to the aquarium. (picture on the right : Hand made glass CO2 injection atomiser by ADA)
Additional CO2 has to be added for these reasons most often in the earlier stages of the aquarium. When there is a lot of bacteria in an older aquarium, sometimes the plants can live off just the CO2 generated by the bacteria, as long as the species and number of plants is suitable.
To find out how much CO2 the plants are consuming, compare the pH levels of the morning and evening. The pH should be at its lowest level in the morning (before turning on the light) after a night of fishes respiring oxygen and expiring CO2, and at its highest level in the evening (before lights out) after a day of plant absorption the CO2 and discharge of oxygen. The greater the difference between these two values, the greater the consumption of CO2, and therefore the greater the health of your plants.
In the daytime, if the pH level won't go down no matter how much CO2 is added, it is because the plants are continually engaged in photosynthesis. The ideal pH level target for aquatic plants hover in the range of pH6.8, but a value of around pH7.5 during the day is not unusual and will not harm your plants or fishes. Rather, potential harm comes from using chemicals that force the pH level down. Potassium carbonate is often used to raise the pH level, but chemicals that work to lower it are to be strictly avoided.