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

An aquarium is an artificial (man-made) and incomplete ecosystem, whereas ponds and lakes are natural and complete ecosystems:

1. Absence of Decomposers: Natural ecosystems like ponds and lakes contain a complete food web, including a diverse population of decomposers (bacteria and fungi). These decomposers naturally break down organic waste, dead organisms, and fish excreta into simple inorganic substances, cleaning the water naturally. In contrast, an aquarium usually lacks a sufficient population of decomposers to clean the waste.
2. Self-Sustaining Nature: Ponds and lakes are self-sustaining and self-regulating. An aquarium is not self-sustaining; the accumulation of uneaten food, fish excreta, and toxic ammonia can quickly reach dangerous levels, harming the aquatic life. Therefore, an aquarium needs regular cleaning, water filtration, and manual water changes to maintain a healthy environment.