Part of modern technical fish keeping is the testing of water parameters. Since so much of the quality of our fishes' lives depend on the quality of their water this makes sense. Those, like myself, who use municipal water supplies have the issue of seasonal "reformulations" of water parameters to contend with. Those who use wells have issues like dissolved gases and metals to worry about. And once we have the water out of the tap and into the aquarium, we need to worry about what the fish do to it. While it is quite possible to be successful without ever owning a test kit, from the standpoint of asking others for help when you have problems, I would say you are on your own, if you can't quote chapter and verse of your water conditions. For me, trying to understand water chemistry, particularly as it relates to the health and happiness of fish, is part of what makes keeping goldfish interesting.
So "I'm game" you say; "what do I need to test for?"
A good question. I am no authority, but in my experience here are some possibilities:
I keep all the frequent and regular testing category kits on hand all the time. The time period suggestions are averages; actual use depends on the conditions. When cycling a tank, you may use "frequent testing" kits several times a day. I found that after I had used the various kits regularly for 6 months (and kept complete, careful notes!) I had a good sense of what to expect and didn't need the kits very often. In essence, I had become calibrated. But I still need to do tests occasionally, since things outside my control can change and affect the water quality.
There are three criteria for selecting test kit products: cost per test, ease of use, and accuracy. All the brands I have used have been sufficiently accurate for hobbyist purposes, so I mainly worry about cost and convenience. I have used Mardel, Wardley, and Tetra products, and have been reasonably happy with all of them. General observations:
My minimum cost recommendation? Do some research on prices, but something like this: Mardel Aqualab 1 (freshwater pH, buffering, and hardness), Wardley Junior Ammonia, Wardley Professional Nitrite and Nitrate. This should run about $25 (as of April 1997).