API's Freshwater Master Test Kit is one example of an aquarium test kit that is easy to use. Photo from API.
One of the most important things to have on hand for an aquarium hobbyist is a master test kit that can test for ammonia, nitrite, nitrate, and pH. Additionally if you plan on keeping fish that are adapted to a different hardness and alkalinity than what your source water is, then you will also need a general hardness and alkalinity test kits. When selecting which brand of aquarium test kit to buy, I recommend you get the test kit you find easiest to use, because a test kit that is difficult to use is less likely to be used.
A pH reading can change depending on the time of day the water is tested. If you test the water early in the morning the pH reading can be several tenths of a point lower than a test taken in the evening before the lights are turned off. The reason for this is because during the night the plants take up oxygen and give off carbon dioxide. Carbon dioxide combines with carbonate to form carbonic acid. Carbonic acid drives the pH down. When the lights come on, plants take up carbon dioxide and produce oxygen, driving the pH up. You should test the pH at least once every two weeks on an established aquarium. A lower than expected pH reading can often indicate excessive nitrate accumulation in the water, and/or low alkalinity.
After an aquarium has been setup for a week with fish it should be tested for ammonia every two or three days. Aquarium ammonia test kit results give a reading that is actually a combination of two chemicals NH3 (un-ionized ammonia) and NH4+ (ionized ammonia known as ammonium) known as total ammonia nitrogen. If any ammonia is detected in the test results you must then test the pH of the aquarium. You then can enter the results of the two tests along with the water temperature in Celsius in the Free Ammonia Part of TAN tool provided previously in this book to extract the toxic free ammonia (NH3). If toxic or near toxic ammonia levels are detected, a water change will be required to reduce the concentration. Once the ammonia concentration in a new aquarium starts to drop without doing any water changes, you can then test every five days until it drops to 0 ppm. In an established aquarium you should not have to test for ammonia unless your fish look stressed, or you have unexplained deaths.