After your tank has sufficiently cycled, testing can help you determine what your water change schedule should be when setting up a new tank, or after adding new fish.
You test your tap water to determine a barometer of what you have to work with,
lets say your tap water has a pH of 7.8, and 0 ppm nitrate, this is your best case scenario, but....
A) You do a water change, take a sample, and your tanks pH is 7.6, nitrate 5 ppm, that may be your normal reality.
B) You "believe" the next water change of 30% will be fine in two weeks because somebody says every 2 weeks is sufficient, so just before that water change, you test.
Your results are pH 6.4, and nitrate of 25 ppm.
To me this means waiting two weeks between water changes is probably too long, and 30% may be too little, because the difference between the results of your pH and nitrate are too great, between A and B.
So you decides to change your water change schedule, instead of waiting 2 weeks, after 1 week. you test, your tanks water has a pH of 7.2, and nitrate and nitrate of 15.
This is a more reasonable drop, but significant enough to warrant a water change.
Test again in a week, if pH and nitrate are similar, this could be a normal schedule.
But then, you go to a LFS and see a few new fish you want, add them, and with the new fish after a week, you pH again drops to 6.6 and nitrate of 20.
To me this means the addition of new fish means, maybe instead of waiting 1 week, you may need to change that water change schedule to every 4 days, instead of every 7.
You also might test your tap water to determine what are the proper fish for your tank.
Lets say your tap water has a pH of 8.2 (Normal in some parts of the US), with hardness of 350 ppm and your tank water has an average of 7.8, and similar water hardness (it hardly ever changes).
When you go to the LFS you see some beautiful wild caught Chocolate gouramis, or Cardinal Tetrasand want them badly .
Google them to see what they need.
A normal pH of somewhere between 4-6, and water hardness of 50ppm for the Chocolate, same parameters for the cardinals.
If you buy them, they may be dead in a few days, and it won't be the LFSs fault.(they may have been keeping them in RO water). A good LFS would warn you though about their needs.
For those water conditions, live bearers and other fish of Central America, and cichlids from the rift lakes of Africa, Rainbows of Australia (not Papua New Guinea) would be better choices.
