Oxygen is already known as a very important substance for all organisms in an aquarium. Oxygen level in a planted aquarium depends on several factors such as:

Water temperature. Cooler water contains more oxygen, that's why you feel fresh after drinking cold water.

Plants' density. Higher density of freshwater aquarium plants produce more oxygen from photosynthesis

Intensity of Aquarium lights & CO2 supply. Unsufficient intensity of aquarium lights & CO2 make fish tank plants can't have good photosynthesis to produce oxygen.

Fish population. More fishes take more oxygen to breathe and produce more waste. More waste will multiply the number of bacteria that need oxygen too to carry out their vital work.

Cleanliness of aquarium & filters. Accumulated waste in the filter media will slow down the water flow & reduce the oxygen supply. Decomposition of dead fish and plants' remnants will also multiply the number of bacteria that need oxygen to do the job.

Degree of surface agitation. Vigorous surface agitation will surely take the oxygen into your tank, but also throw away other gases including the precious CO2 that needed by fish tank plants (gas exchange process).

One clear sign of an unsufficient oxygen is fish gasp for air at the water's surface. A fish with no labyrinth can't take oxygen directly from open air, it will die soon if there's no effort to add dissolved oxygen into the water such as aeration.

In the past, CO2 was believed as a toxin for fishes that should be thrown away from aquariums by using aeration, vigorous surface agitation, venturi device or splashing the output water from the filter.

Now, CO2 is known as an important nutrient for freshwater aquarium plants. If some hobbyists claim that they can keep fish tank plants without any additional aquarium CO2 system, it might be true if they are only keeping slow-growing & low-light fish tank plants such as Anubias, Microsorium, Criptocoryne wendtii, Javanese fern or Echinodorus amazonicus. These fish tank plants don't need strong light & consume a small quantity of CO2, so the need for CO2 may be fulfiled only from fishes' respiration and the plants' own respiration at night. In natural habitats such as river or lake, CO2 supply comes from ground water & ground fermentation. Ground water is actually rainwater which contains rich of CO2 that seeped through the soil into various deep rock & earth layers, and some finally come out as springs.

In a planted aquarium with many varieties of freshwater aquarium plants, an adequate supply of CO2 is essential as the main nutrient, so both the fast-growing and the slow-growing freshwater aquarium plants will thrive. An aquarium CO2 system with automatic control will balance your CO2 need with its interdependence parameters (pH and KH/Carbonate Hardness values), so you can sleep well and let the device do the important job for you.

O2 & CO2 interdependence

Freshwater aquarium plants have a daily cycle with two main activities : photosynthesis (at day) and respiration (at night).

In photosynthesis, fish tank plants use the energy of aquarium lights and CO2 to synthesize nutrients from the ground and water, oxygen is produced as a waste product and released by the fish tank plants into the surrounding water. In a well-planted aquarium with adequate lights & CO2 nutrients, there is always sufficient oxygen available for all organisms.

In respiration process at night, freshwater aquarium plants consume oxygen and produce CO2 as a waste product. So during the respiration process, oxygen level will decrease, CO2 level will increase and reduce the pH level. In the early morning when photosynthesis not yet begin, pH is at the lowest level. It will increase gradually during the photosynthesis process when the fish tank plants consume CO2.

Is it dangerous for other organisms that consume oxygen all the time, especially during the night? Actually, healthy fish tank plants produce far more oxygen during the day than they consume at night, but again another question arise : Is the remaining oxygen enough for all other organisms?

The answer is depend on the other organisms that you keep. If you have a dense fish population, maybe the oxygen won't be sufficient for the fishes' respiration during the night. Before your fishes gasp for air at the water's surface, it would be better if you check your oxygen level at night by using water testing kits - normal oxygen level is about 5-8 mg/liter (ppm). If the oxygen level is below the normal level, you have 2 options :

Reduce the number of fishes and other organisms.
Add an oxygen into your tank at night by using aeration, vigorous surface agitation, venturi device or splashing the output water from the filter.

If you choose the 2nd option, then you will face a dilemma : any above actions to add oxygen will throw away the precious CO2 for the next-day photosynthesis!. So, what should you do?

Read some solutions in our next article (O2 vs CO2).