Other than the abiotic components, which of the given biotic components are not required to make an aquarium with small herbivorous fishes a self-sustaining system?

(i) Aquatic plants and aquatic animals

(ii) Terrestrial plants and terrestrial animals

(iii) Decomposers as bacteria and fungi

(iv) Consumers as clown fishes and sea urchins

As per the NCERT chapter, an ecosystem (such as an aquarium) consists of biotic components (living organisms) and abiotic components (physical factors like temperature, water, minerals). For the biotic part, the chapter states that organisms are grouped as producers (green plants, which make food by photosynthesis), consumers (herbivores, carnivores, omnivores, parasites, which depend on producers directly or indirectly) and decomposers (bacteria and fungi, which break down dead remains and return simple substances to the soil).

For an aquarium with small herbivorous fishes to become a self-sustaining system, the chapter's Activity 13.1 makes clear that adding a few aquatic plants and aquatic animals makes it self-sustaining — aquatic plants act as producers (releasing oxygen and food) and the herbivorous fish/aquatic animals act as primary consumers. Decomposers (bacteria and fungi) are also essential biotic components — they break down waste and dead matter, recycling nutrients back for the plants (without decomposers, natural replenishment of nutrients does not occur, as the text explicitly asks the reader to consider).

What is NOT required:

• (ii) Terrestrial plants and terrestrial animals are land-based organisms; they do not belong to the aquatic ecosystem of an aquarium and cannot survive or function in it, so they are not needed.
• (iv) Additional consumers such as clown fish and sea urchins are marine organisms not suited to a small freshwater aquarium with herbivorous fish; moreover, Activity 13.2 explicitly warns against adding an aquatic animal that would eat others (a predator), so extra higher-level consumers are unnecessary and could destabilise the balance already provided by the herbivorous fish.

Hence, options (i) aquatic plants/animals and (iii) decomposers ARE required (ruling out A, B, C), while (ii) and (iv) are NOT required, making D the correct answer.