An ecosystem is defined as a place where living and nonliving things live together interacting with each other for the sustenance of life. The two main components of an ecosystem include biotic (living) and abiotic (nonliving) components. Ponds and lakes are natural ecosystems where many organisms live. Thus in these ecosystems, toxic waste (ammonia) produced by living organisms is converted into the harmless form by the activity of decomposers. An aquarium contains only fishes and no decomposers. Therefore, it is necessary that the toxic ammonia produced by the fishes and other wastes should be regularly removed by changing water in an aquarium.