08-08-2010, 07:28 PM
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One important point missed so far is that water changes are usually absolutely essential and have very little to do with the nitrification cycle (which is what ammonia, nitrite and nitrate relate to).
First, let me mention that in a heavily-planted aquarium with a moderate fish load, the plants will take care of things. But a regular weekly water change is still not a bad idea, just in case; sort of like insurance.
Leaving plants out of the picture for the moment, water changes depend upon the fish--what they are and how many--in relation to the water volume.
In nature, no fish lives in the "same" water for more than a second. Water is always changing--streams, creeks and rivers flow even if slowly, and in lakes and ponds there is water movement through thermal stratification and the percentage of fish to water volume is vastly less than in any normal-stocked aquarium. Same holds for streams, etc. The fish are constantly in "new" water. And that water itself is being "changed" by nature--rain and snow melt, evaporation (which is very high in the tropics), etc. If the fish don't like something in the water, they swim away somewhere else. That option is not there in the aquarium, they are truly captive, forced to contend with whatever we create. And water is far more significant than air in this respect. An animal in a cage has constant circulating air; a fish in an aquarium is in a confined space that is only going to improve if the aquarist does a water change.
Fish release pheromones into the water, and some of these affect other fish in various ways. Waste products are broken down by bacteria, but they remain in the water in that state until we remove them. And no one who performs weekly water changes of 40-50% can fail to notice the improvement in the fish's activity and behaviour after each one.
"Perfect" water does not exist in any aquarium. No filter can remove the above substances; filters only circulate the same old water around and around. Nitrification bacteria colonize surfaces and handle the measurable nitrogen states--having zero ammonia, nitrite and nitrate is fine, but that is only one part of the story. You cannot measure the above substances, but they are there, adversely affecting every fish in the tank.
Those aquarists who only do a water change when nitrate is high are already doing too little too late. By the time nitrates reach "high" levels the fish have been weakened by what is causing them to rise.
A weekly water change is crucial to the health of an aquarium. The more plants and fewer fish, the fewer water changes because we are allowing nature to do what it does best, but this is very limited in our aquaria. The volume of water changed weekly will depend upon the fish and aquarium capacity, and plants too. For 15 years I have changed 50% of my tanks every week, and I have several to do. But I would not jeopardize the health of my fish by failing to do this task.