It is advisable to keep green plants in an aquarium.

Green plants carry out PHOTOSYNTHESIS, using the carbon dioxide released by the fish and giving out OXYGEN, which the fish need for respiration. The plants also absorb some of the nitrogenous waste as nutrients, keeping the water cleaner.

The reason must name the EXCHANGE, since that is the whole point: the fish and the plants supply each other’s needs. Fish use oxygen and release carbon dioxide; plants use carbon dioxide and release oxygen.

This is a miniature version of the global relationship between plants and animals — which is why examiners like the example.

ONE CAUTION worth knowing: plants photosynthesise only in LIGHT, but they respire day and night. In darkness they consume oxygen rather than producing it, which is why a heavily planted aquarium needs light as well as plants.