Mike, the amount of water changed at each water change (which should always be weekly) depends upon the tank size, the fish--type, size, number--and whether or not there are live plants. The more fish or the larger the fish the more water should be changed. Live plants do a lot of filtration work and with a moderate fish load, few (some suggest no) water changes are needed. Most of us tend to have more fish than "moderate" so a water change is a good idea even with plants. I do 50% weekly and have done for more than 15 years, and I have fairly heavily planted tanks, but I also have quite a lot of fish in them.
The purpose behind the water change is to remove the "crud" that otherwise remains in the water. I borrow the term "crud" to refer to dissolved fish waste (the liquid), pheromones released by the fish, and other biological stuff. Filters move water around, and bacteria handle ammonia and nitrite. But filters only circulate the crud around. In nature fish do not live in the same water for more than a few seconds. Water is constantly flowing past them in streams, and even in lakes there is water movement and the ratio of fish to water volume is so minimal that the fish is always in different water. The only way to even begin to approach this "ideal" state is with a water change. It would be better for the fish if we could do daily or even hourly water changes--but that is not feasible for most of us; those raising fry like discus frequently do several major water changes each day.
Some will advocate a water change only when nitrates are high, or pH drops (acidifies). But by this time t is far too late, the damage has been done. Deteriorating water conditions is a sign that something is seriously wrong biologically. A weekly water change sufficient for the aquarium will maintain stable water parameters. Over the course of months and years, the pH should not fluctuate (aside from the normal diurnal shift common in nature and in aquaria), nitrates should not rise above the level they are due to the biological stability unique to each aquarium, and ammonia and nitrite remain zero. Regular water changes actually work to achieve this goal. The parameters of the source water should not be too far off from the aquarium water; if there is a significant difference other issues can result from larger water changes. The hardness of the water has a role in this equation.
Last edited by Byron; 09-24-2010 at 07:41 PM..