03-16-2013, 08:43 PM
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This is not really an easy question to answer. As you will note with those who have responded, most of us have a fair number of years experience. And it is true that we can probably look at the fish in the aquarium and within seconds know if something is wrong. That ability in my view eliminates the need for tests, generally speaking.
The tank volume, fish stocking, live plants, and regular water changes also factor in to this. With live plants, not overstocking, not overfeeding, and performing weekly partial water changes of 1/3 to 1/2 the tank you should never run into issues with ammonia, nitrite or nitrates [assuming these are not present in the source water].
In a balanced aquarium, the water parameters (GH, KH and pH) should remain stable. The GH and KH will be close to the tap water. The pH may lower naturally, depending upon the biological system and the KH of the soource water. Testing this weekly just prior to the water change for a few weeks is a good idea until you see it is stable.