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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.
 

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I think 1077 has summed it up best in the last post; regular partial water changes keep healthy fish healthy, and that's what it's all about.

The reaction/behaviour of fish when the water is regularly changed clearly demonstrates that there is probably a lot more benefit than anyone can measure in nitrate levels or hardness or whatever. A closed system can alter in ways we may not recognize however often we test for this or that. To gammahermit, I would never suggest only changing water due to certain test results, that in my view is meaningless, because there are all these other factors in the equation.

An aquarium is a closed system (aside from the exchange of oxygen/carbon dioxide at the surface) that does not exist in nature. Water changes are important in maintaining a healthy aquarium as it is the closest we can come to creating a natural environment for the inhabitants.
 

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I would agree that periodic water changes are necessary for the fish to flourish but what I was hoping to get out of this thread was 1st understand why they are necessary & 2nd to figure out how often it is necessary rather than to just say every week or every month.

I would have to disagree with Byron that nitrate & hardness measurements are meaningless. But instead say they are insufficient to determine when a water change is necessary. And that with enough information it can be determined what the maximum time between water changes is.
Sorry, my fault for not making it clearer (that's the trouble with emails, we can't see what we're each thinking when we write words!). Tests for nitrates and hardness and pH and... are obviously important. But the time at which you do a water change should not be determined by test results. That's like saying you will only do a water change when something is wrong (the nitrates are suddenly higher, whatever). That is not good aquarium management. As I think several of us have indicated, regular water changes have an obvious effect on the health of the fish, and that is the bottom line. And everything I've read indicates changing less water more often is much better than changing more water less often. There is a real biological issue here.

The aim is to maintain consistent water quality permanently. This is not as impossible as some may think. There are fluctuations that occur naturally, in nature and in our aquarium. Fish have evolved to adapt to these minor fluctuations. This involves temperature and pH (there are diurnal fluctuations both in nature and in an aquarium), hardness, and dissolved organics in the water. Fish are very closely tied to their environment. As an example, fish take in water through their cells by osmosis. The fish must adjust its internal pH to equal that of the water passing into its cells. In an excellent article on "Fish Growth vs. Tank Size" in the December 2006 issue of TFH, Laura Muha notes that "Both salinity and pH affects a fish's growth rate because they affect how hard a fish's body must work to maintain its physiological equilibrium--that is, the complex chain of internal chemical reactions that keep the pH of its blood steady, its tissues fed, and its immune system functioning. When pH and/or salinity stray outside the ideal range for any given species, the fishes' bodies must work harder and use more energy to maintain this equilibrium." We all know how stress affects humans, and it is now known that this occurs with fish as well. Having fluctuating water conditions means the fish is constantly having to adjust its metabolism, and this stresses the fish and can lead to poor health, disease, and even death if not corrected. The point of regular water changes regardless of what the water conditions may be is that it is establishing an equilibrium in the tank and therefore in the fish, resulting in healthier and happier fsh. Changing 25-40% of the water every week is maintaining such a balance. This answers your questions as to why and how often.
 
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