Something I like to try to get a handle on. In reading I can see that there is a wide range of opinions on water changes. How much should be changed, how frequent and what are the factors that help make those decisions. In one of my other thread someone made the point that every tank and every person is different, and what works perfectly well for one tank might not work as well for another depending on inhabitants, environment and other factors. So I guess what I like to ask here is, what is accomplished by the water change? Hopefully that will help me see the reasons for some of the differences of opinions. One thing I know that the water change will do is reduce the number of nitrates in the water. If you are replacing water that has been in the tank for some time with water that is fresh, whether it is Ro/DI water or tap water. This dilutes the nitrate level. Nitrites and ammonia should be unaffected because in a well-timed cycle tank those are being removed by the bacteria as fast as they are being created. This would suggest that the frequency and amount of water being changed would be what is necessary to keep the nitrates at an acceptable level since they will not be eliminated. In this case, planted tanks would need (theoretically) less water changed and/or less frequently because the plants would absorb some of the nitrates, slowing the rate at which it builds in the tank depending. This would also mean that bio load would matter, with many fewer fish the same tank would need to be changed less often. If this is the primary governor of water changes, how much is too much nitrate? Another thing you can obviously do is clean the substrate/sand/whatever you have at the bottom of the tank. My first water change with that was somewhat humorous, I either was not getting any of the substrates circulating in the larger part of the vacuum to or I was filling the whole thing up! I can tell that is can take more practice. The question again becomes what are we cleaning, are we removing wastes so that they will not go through the nitrogen cycle, thus reducing the amount of nitrate that we eventually have? Are we getting rid of other debris? If this is the primary driver of water change, it seems like the amount of water would matter less than whether or not you are able to clean most of the substrate. In this case the frequency should be related to how dirty the tank gets, which is probably also related to the bio load and perhaps the specific inhabitants. I know there are factors, such as getting ich, that would cause a change in the normal rates. Anyways, I was hoping that some of the veterans or more experience beginners would talk about what they see as the purpose of the water changes that they do. I assume that you will talk a little bit about your frequency and amount of water you change in explaining the reasoning and what you are trying to accomplish, but I am primarily looking at the reasons and not just the amounts and frequency. As I already stated, I know there is a fairly wide range of opinion there.