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

Good question, But reason is simple.... Ponds laked.. etc are at hand of nature whereas at our hosuse we need to clean and protest theie live so./
You dont have to change water or clean an aquarium if you have soil and good amount of plants in it. You would just have to refill the evaporated water. Cleaning the tank once in a month will still benefit the fishes..... (saying frm experience)


These places should be cleant to preserve fishes form deing
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but why pond do not fishes are also there?
Who told you that we dont clean lakes and ponds?? 
the municipality cleans the ponds and lakes with the taxes that we pay.
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can it be answered relating to ecosystem?
no, because your question is wrong..they are cleaned
how it can be wrong it have been asked in the board paper of 2012
sorry its 2010
which subject?