04-05-2009, 08:10 PM
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Mikaila31 really said it very well, and I'm only going to emphasize it a bit because I'm concerned that you may be contemplating not doing partial water changes and that spells disaster in time.
Fish take in water through their cells (called osmosis) and expel it as urine after they have used it to do biological processes much the same as we do. In time, they would literally be swimming in a tank of urine. Not healthy for fish any more than for us.
As Mikaila31 said, fish need minerals and that has to come from water changes because they use up the minerals in the existing water fairly quickly. If it is a planted aquarium, the same happens with plants. The nitrates produced as a result of the nitrobacter bacteria do build up if not removed; mosat aquarist aim for nitrates around 20ppm or lower; at over 40ppm they are toxic to many fish. Plants utilize some of it through the action of the bacteria in the substrate, but if there are anaerobic bacteria in the substrate (as there usually are in varying numbers) they produce nitrogen gas and hydrogen sulphide. More toxins that need to be got rid of.
There is a small school of aquarists (I suppose there still are, I haven't come across any on this forum) that advocate it is unnecessary to do water changes in a thickly planted tank with few fish. I am skeptical of their claims, and would not want to risk it. Besides, like most of us I like to have lots of fish in my tanks, and without water changes I certainly wouldn't have them healthy for very long.
It is no surprise that fish will often spawn immediately after a significant water change. I know there are a number of reasons why this occurs, but the point is that without the water change they would not be motivated to spawn to the same degree. It's natural, and as Mikaila31 said, they like it.