Oh i have done nitrate test at beginning of cycling, for comparison sake it was 5.0ppm on day 1 and 5.0 again on day 21 but to be honest i'm not sure what nitrate signifies, or how high is unwelcome
Once our tanks have cycled and the filter has matured and settled down, there are two main reasons for performing water changes.
Firstly, if you have soft water like Sue, you need to replenish the trace elements that cause hardness in water. These are called "buffers" and they prevent large and sudden changes in acidity / alakalinity. These are called pH crashes and the sudden changes are very harmful to fish.
Secondly, water changes control the level of nitrates in the water. Nitrates are the end-product of the filtration systems in your tank. Ammonia is converted to nitrites, nitrites are converted to nitrates. Left alone, the level of nitrates would continue to rise over time. At "high" levels, these are harmful to our fish.
Now one of the problems we face here is that no-one seems to agree on what "high" levels mean. The guy Sue mentions who reckons that "High" means more than 10 is living in a fantasy world; My nitrates, from the tap, measure 35-45! So I am poisoning my fish every time I do a water change apparently! Others say "50" is high, some, like Sue, reckon your tap levels +20 or so. In theory, this is a metric that anyone can reach.
Sue is generally pretty pragmatic about these sorts of things; however, and I'm going out on a limb here by disagreeing with Sue (Please no hate mail), It would be nice if Sue could explain how I can measure the nitrate in my tank and be able to tell the difference between 65, which is a "safe" level, and 100 which is an "unsafe" level. The tests we use are pretty unreliable and, not having a spectrometer to hand, almost impossible to grade with any degree of accuracy.