A student listed characteristics of a classroom aquarium ecosystem. Which
characteristic is a biotic component of the aquarium ecosystem?*
The water temperature is 23 degrees Celsius
O
The pH of the water is between 6.5 and 6.8
O
The ammonia is controlled with a filter that uses two type of bacteria,
The lighting is supplied by natural sunlight and a fluorescent light bulb.

The Biosphere on earth is composed of all the ecosystems, while each ecosystem is composed of the abiotic and biotic components and their mutual interactions. Aquarium is an example of aquatic ecosystem.
Abiotic components constitute the non-living part of the ecosystem.
Biotic components constitute the living part of the ecosystem.
In the aquarium,

The abiotic components include the air, water, heat, minerals, sunlight, fluorescent light.
The biotic components include the aquatic plants, aquatic animals and the bacteria (in the filter attached to the aquarium).

These bacteria are responsible for fixing dissolved nitrogen gas present in the water of the aquarium to ammonium and are called nitrogen-fixing bacteria.
Free living bacteria like cyanobacteria or blue-green algae are examples of nitrogen-fixing bacteria.
Fixation of dissolved nitrogen gas to ammonium and then conversion of ammonium to nitrites and nitrates (done by other bacteria) is essential because plants (terrestrial or aquatic) can use only nitrates as the source of nitrogen for the purpose of metabolism.