Photosynthesis is the process by which plants use chlorophyll, with the aid of light energy, to synthesis water, carbon compounds, and other nutrients into sugars.
These sugars provide energy for the plant’s growth. In water, carbon dioxide (CO2) dissolves quickly and is therefore the simplest carbon compound for plants to utilise. Thus, aquatic plant life depends on carbon dioxide in the water in order to survive.
As photosynthesis requires light, this process can only occur during the day. In this period, plants draw carbon dioxide from the water and release oxygen into the water.
At night however, there is no photosynthesis and the reverse occurs – plants consume oxygen and release carbon dioxide.
In the average aquarium, light levels are usually low and the amount of CO2 produced by the respiration of the fish during the day is barely sufficient to allow some plants to photosynthesis and grow.
However, many plants require more light than is generally provided and it is likely that with an increase in light, there will not be enough CO2 in the aquarium to facilitate photosynthesis. Put simply, the plants cannot grow as fast as they would like to, given the available light energy.
The importance of light in an aquarium
Light is the first determining factor for photosynthesis and growth.
As light increases, plant growth will increase. As plant growth accelerates, CO2 will be used faster and additional quantities may be required. Carbon dioxide without sufficient light will reduce photosynthesis and cause an excess of CO2 to be dissolved into the water.
A balance must be maintained between light levels and the amount of CO2 being injected into the aquarium in order for sufficient levels of photosynthesis to take place.
Increasing the population of fish (which respire and therefore increase the amount of CO2) may subsequently provide just enough carbon dioxide for the plants in the tank, but the consequences of overstocking the tank often outweigh any benefits.
A large population of fish can produce tremendous amounts of ammonia and nitrogenous by-products.
Both compounds are usable by plants, but an excess of them can cause uncontrollable algae blooms and prove to be toxic to the fish. Additionally, as the carbon dioxide levels increase, the dissolved oxygen levels decrease (but not to the complete exclusion of oxygen), making it difficult for the fish to breathe.
Aquarium carbon dioxide system
A way around the overstocking problem is to inject carbon dioxide directly into the aquarium.
Anyone who has witnessed the rapid growth of aquarium plants in response to carbon dioxide (CO2) fertilization must be convinced of the usefulness of this system.
Sure, there are thousands of aquarium hobbyists who do not give their plants any sort of special treatment yet still end up with a fairly nice display.
However, truly luxuriant growth, the sort that you see on the covers of fish keeping aquaria magazines can only be achieved by fertilizing with CO2.
In order to maximize the benefit of injecting CO2 it is important that you reduce any surface turbulence, as this will quickly allow dissolved carbon dioxide to escape into the air.
This is one reason why people find it difficult to grow plants if an undergravel filter is employed – the carbon dioxide is driven from the water.
As a guide, a good level of CO2 in an aquarium is around 35 parts per million (ppm). This can be checked with a carbon dioxide test kit or by simply observing the behavior of any fish you may have in the tank.
If you see your fish “gasping” at the surface or breathing rapidly, the level of CO2 may be too high (oxygen too low). Levels of 10ppm CO2 suggested by many, stem from research with coldwater trout and similar fish and have been carried across to tropical fresh water applications over the mists of time.
CO2 concentration must exceed 50 ppm before becoming dangerous to fish (however, some species such as Guppies can tolerate values as high as to 750 mg/l) but the pH drop caused as carbon dioxide levels increase is more concerning. This is because as carbon dioxide dissolves it forms carbonic acid.
As mentioned earlier, during periods of light, plants draw CO2 from the water and at night plants release CO2. Therefore, your light and CO2 injection system should work together.
When the lights are on, the CO2 injection system should also be operating. When you turn the aquarium lights off, you should deactivate your carbon dioxide injection system to prevent an excessive quantity of CO2entering the aquarium.
It is however important that pH levels do not fluctuate excessively at any time, day or night, and an electronic pH controller can be a useful addition to a carbon dioxide injection system.
CO2 injection makes it easy to grow aquarium plants, but it is not a cure-all. You still have to observe some of the other essentials of proper plant care.
Aquarium plants need a lot of light additionally, iron fertilization is beneficial for maximum plant growth and above all a balance of light, oxygen, carbon dioxide, pH and nutrients must be maintained.