There were too may responses to quote for this one (LOL) but I'll give you the explanation I was given for changing water:
In a closed system, chemical compounds we test for and don't test for accumulate in the tank. By doing partial water changes frequently, you eliminate or reduce these compounds from reaching higher, more toxic to the fish levels. That's the reason to do water changes.
As for the When and How Much: Fish tend to like and do better in a steady more balanced environment. By doing smaller, weekly water changes, the fish tend to not feel the difference of the change with the new water. If you are starting out with no to low chemical buildup and you replace some water with low to no chemical buildup, the fish tend to respond better. ( We called this "sweetening" of the water.) Dieoffs and illnesses occur more often when massive changes are done to the environment. In the wild, fish will move to another area where the water is more to their liking when the environment gets too bad. Often, during the rain, fish will move to more sheltered areas until the rain has stopped. Another factor from the rain is the oxygen content of the water. Rain will add a higher oxygen level to the water from all the surface aggitation. Sometimes, it's even too much so the fish wait until the water has settled down before returning to the area. (This is also true when fishing , my other past time
) Since the fish in a closed system can't leave, it's best to keep their environment as stable as possible. Doing weekly smaller changes can achieve this. You COULD do it daily but to me, that's overkill. I have other things to do with my day too
( and my fish don't seem to mind it.
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There's a separate, less talked about reason for smaller changes: YOU DON'T CONTROL THE WATER. For those of you using tap water with chemicals, do you know that your public water companies can change or alter their chemicals in the water without telling you? We had a situation back in the 1980s where our water company switched to chloramine without telling anyone. The pet store I was working in at the time suffered massive losses of our fish because our filters were just there to take out chlorine. The ammonia killed the fish. Now, take that on a smaller scale, if you added a toxic chemical to your tank, but it was in a low dose, chances are your fish would survive the addition until your chemical filter removed the toxin. If it were added in a large dose, it wouldn't bode well for the fish.
Using Nitrates as a guide is really a haphazard way of gauging. If you have live plants in the tank and they are keeping the nitrates low, they aren't necessarily reducing the other chemicals of the tank so you can still have a toxic environment even with low Nitrates. Even planted areas in the wild get rained on to help flush out the area. I'd venture to say that if water were tested in areas high in fish life and void of fish life, the second area would have foul water. To the poster who said their fish were better after doing large changes, I'd venture to say that the fish are doing better because the pre-change environment is too toxic or extreme too frequently and not balanced. Less fish = less buildup= less extremes for longer periods= healthier fish.
To me, having a nitrate problem in less than a week means the same thing, too many fish in too small a volume of water. I've kept fish for many years without ever testing for nitrates because I never let them get to any level of concern. The way to tell if this is a proper method is to judge their size and color vs their wild counterparts. If a wild fish grows to, for example, 5" and your fish only get to 3" in your tank, that should tell you that something is wrong. More often than not, it's environmental.
Here's something to think about: If your fish do better after a water change, doesn't that mean that the water they were in before the change was not good for them? I prefer my fish to not do better but do the same after a change. That means that I am keeping a healthy balanced environment for them to grow in.
That's my 2 cents ( but I paid a lot more than that for this education
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of natural occurrence.
