I recently bought a water test kit which I use to monitor the most important water quality parameters. The most important parameters important to the wellbeing of the fish are; PH, Ammonia, Nitrites, and Nitrates.
To measure a specific parameter you add a small quantity of the water in the test tube and add a few drops of the appropriate regent. The water changes color which you then compare to card to take the reading. For instance the ammonia reading in the attached image is about 0.25 ppm (parts per million).
PH is the acidity or alkalinity of the water and the reading should be neutral about 7.0. Ammonia and Nitrites should be close to 0.0ppm. Ammonia and nitrites are very toxic to fish. Readings above 0.0ppm may indicate too many solids in the system or that the bacteria is not working effectively. Nitrate reading can be as high as possible. Nitrates are good for the plants and their presence indicates the nitrifying bacteria are working well.
So there you go! A water test kit is an essential item in running your system effectively.