Testing your water should not be made as complex as you apparantly have made it.
Buy yourself a good complete test kit, I use this one
http://petsolutions.com/product.asp_Q_pn_E_17101034_A_Liquid+Master+Test+Kits_E_ from aquarium pharmacuticals. It tests Ph Gh Kh Nitrate Nitrite and Ammonia, and should be everything you need for a freshwater setup.
The best plan of action is to take a 3 tiered approach to water testing.
1) Test your tap water every 2 or 3 months. This way you will know what the parameters of your incoming water are. If you are using a municipal water source, it can vary from time to time depending on what they are adding to the water. Always keep an updated log as to what your tap water parameters are.
2) Know what your fishes want. Do research and find out what the optimal water parameters are for the fish you are keeping. If they are WAAAY off from what you can provide, you may want to rethink the types of fishes you are keeping. An example would be, if you wanted to keep Discus and your tap water was 8.4 in pH or if you wanted to keep hard water cichlids and your tap water was 6.8 pH. Each example requires special and potentially expensive methods of providing water alteration (RO water, crushed coral, etc.) that may be too much for you to affordably or feasably handle.
A pH extreme AND a Gh extreme are a lot harder to compensate for than just one or the other.
3) Water tests should be done before water changes, but may also require testing after the water change as well.
Ideally you will want to know what the water looks like before you change it so you will know what is going on between water changes. An example would be testing your water before a water change and finding an ammonia, nitrite or nitrate spike. Finding a relatively high spike in either category may entail a different approach to your scheduled water change. Ie. if you had a ammonia spike, you may want to do a larger water change than if you didnt, or if you had a very large nitrate spike.
Another point to consider is if your tank water shows a pH of 8.0 and your new water contains a pH of 6.8. You would NOT want to do a large water change of 50% or more in this instance because you would greatly increase your risk of osmotic shock by lowering your pH too rapidly.
Idealy, if old water and new water are vastly different in pH Gh or temperature, you want to make smaller water changes so as to not "shock" the fish.
Again, idealy pH changes should be done over a long period of time and should not exceed .2 up or down in less than a 24 hour period. Gh is not near as crutial, but again, HUGE variants between incoming and outgoing may require special attention. Temperature falls somewhere in the middle. Ideally, new tank water should be within around 5 degrees COOLER than the water it is replacing. Cooler water heats up quicker than hotter water cools down.
3a) If you are attempting to change water parameters (ie. you are trying to adjust your pH or Gh) you should then retest your tank a few hours after the water change. When testing after a water change, you will want to give your new water time to circulate around with the old water. (This time will vary depending upong the amount of filtration or circulation your tank has.) You will want to know what effect, if any, your water change had on your tank. This would be especially true if your incoming and outgoing water parameters were very different.
The water samples you take for testing purposes can come from any part of the tank. There doesnt need to be any special "place" from which you collect water. I usually take a very clean glass and skim about 1/2 cup of water from the top of my tank. I then use a syrengie to transfer the water from my glass to my test tubes.
I hope my post didnt sound too confusing, but I tried to give an overall example of how and why to test your water.
NitrAtes in the 20ppm or lower range are not believed to be harmfull to most fishes. If they exceed 20ppm they can prove fatal if fishes are exposed long term. Ideally, you will want to keep your tank at 10ppm or below.