08-23-2009, 09:47 AM
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The partial water change (pwc) accomplishes what no filter can--changing the water. Fish constantly excrete waste, liquid and solid, into the water. Filters and bacteria can break down the solid into liquid but they don't remove the liquid (plants will do some, but not sufficient for the number of fish most of us have in our aquaria). The other big plus in pwc is diluting nitrates. This is less important in a planted tank since the plants use the ammonia and nitrite before it has a chance to be converted by bacteria into (finally) nitrates. Those with planted aquaria usually see nitrates well below 10 ppm, mine run around 5ppm.
In nature, fish do not live in the same water; water is constantly moving past them at varying rates. We cannot duplicate this in an aquarium except by changing part of the water. The more often the better, and changing less more often is better than changing more less often. Daily would be great, but most of us can't manage that, so weekly is considered to be the best schedule. The amount depends upon stocking level (fish), plants, size of aquarium, and type of fish (larger fish produce more waste so the pwc has to be larger). I do 40%-50% every week without fail, and have for 15+ years.
The aim is to have more consistant water quality and water parameters. More frequent pwc achieves this better than less frequent. The fish will thank you.