Which of the following are called secondary consumers?

We do not clean ponds or lakes,but an aquarium needs to be cleaned. Why?

The length and complexity of food chains remain mostly the same.

Suggest one word for each of the following statements/definitions:
$(a)$ The physical and biological world where we live in
$(b)$ Each level of food chain where transfer of energy takes place
$(c)$ The physical factors like temperature,rainfall,wind and soil of an ecosystem
$(d)$ Organisms which depend on the producers either directly or indirectly for food

The flow of organic matter in a food chain is unidirectional.

What is meant by biological magnification?