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 can not
grow as fast as they would like to, given the available light energy.
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.
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 maximise 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 behaviour 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.
Remember, while carbon dioxide is good for plants, excessive
amounts, reducing Oxygen levels and lowering the pH, can kill your fish. You
should balance the needs of all the organisms in your tank. If the CO2
levels in your tank are excessive, vigorous aeration of the water with an air
pump will quickly correct the problem. By aerating your tank with an air pump,
you allow CO2 to escape and be replaced by oxygen. Refer
to the relationship between Hardness, pH and CO2
here.
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 CO2 entering 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. Electronic pH controllers monitor the pH and regulate
the carbon dioxide injection in order to maintain a constant pH (and CO2 level) 24 hours per day. Since the plants produce CO2 at night, a lesser amount of CO2 will
be injected by the automatic system and a night than during the day.
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.
