Nitrates 40 ppm and higher are dangerous to the fish. NitrAte is the least harmful form of ammonia in the tank, but in high amounts it is harmful. If your tank is cycled NitrAte is the only one you should test on a regular basis. That being said I do not test NitrAtes unless I have a reason to believe there is something wrong with the water. I have a routine for water changes, and I do them before they are required by testing results. I do weekly changes of 25-50 percent on all of my tanks. I do my 10 gallon guppy tank twice a week, as I have fry and want to ensure I have no mini cycles with the additional babies adding to the bioload. Keep in mind if you cant find the test, and cant afford to order it online, you should be able to take the water to your LFS for testing. Make sure you ask for the number results as this is how you will find out when to set you water change routine.