Which of these is an abiotic factor of an aquarium ecosystem?
a. algae growing on the glass
b. gravel on the bottom of the aquarium
c. plants growing from the gravel
d. fish swimming in the water

b. gravel on the bottom of the aquarium

An ecosystem involves both the biological (plants, animals, human beings) and non-biological (land, water, soil, and atmosphere) community which interacts as a system. More importantly, the living things are very dependent on the abiotic community since it cannot survive by itself. Every animal, plant and human needs the primary physiological needs of water, food and shelter provided by the abiotic system.   

Biotic factors:
Plants
Microorganisms
Macroorganisms