(a) An aquarium is an artificial and incomplete ecosystem compared to natural water bodies which are self sustaining and complete ecosystems where there is a perfect recycling of material. In a pond or lake, the waste produced serves as the food material for the decomposers and is thus eliminated by them from the water. Whereas in an aquarium, the uneaten food and waste generated mixes with the water due to the lack of decomposers. Therefore it needs to be cleaned.
(b) the microorganisms like bacteria and fungi which breakdown the complex organic substances into simple inorganic substances that go into the soil and are used once more by the plants are called decomposers. If all the decomposers were eradicated from the ecosystem, then there will be no agency to decompose dead bodies of animals and plants. They would start giving foul smell and it would become impossible to live on the earth. It is the decomposers who help to replenish the soil by their conversion of organic into inorganic compounds. In the absence of decomposers, natural replenishment of soil does not take place.
