we do not clean ponds but an aquarium needs to be clean why?
Aquarium is just an artificially created ecosystem, while ponds are more complex and ecosystem-like. Each ecosystem should contain 3 types of organisms: producers, consumers and decomposers. In aquarium: green plants (producers) assimilate carbon dioxide and mineral compounds while giving the O2. Oxygen is required to fish (consumers). Fish produce terminal metabolites (NH3 etc.) that are removed by decomposers. Upon breathing all organisms produce carbon dioxide that is required by plants. But there is a problem with fish feeding, since it’s not enough eat for them (only some algae as usually). This problem is solved by adding the food to fish from outside. There are also some other problems that are solved with human help. For example, humans should add mineral substances for plant growth, periodically change water etc. With old toxic metabolites are removed. Decomposers of aquarium aren’t able to maintain the whole system in clean state, sine their diversity is too small.. Ponds usually have larger species diversity in comparison with the aquarium. There are more sources of feed for fish, more species of decomposers etc. All species are in balance with others. So, ponds are small ecosystems that don’t require human help.