The chemistry of your aquarium water should not be a mystery to you. Your fish depend upon you providing them with a balanced ecosystem. While it's possible, with careful and patient preparation, to maintain a healthy aquarium without testing the water, the first sign that something has gone wrong will most likely be a dying or dead fish.
Fortunately, the task of measuring the health of your aquarium has become much easier in the last few years as testing equipment, once only available (and affordable) to professionals, has become relatively inexpensive and common in aquarium shops. The equipment comes as simple test strips that change color, in electronic probes that give digital readouts, and even some that take constant readings and make adjustments in your equipment. For most tanks, the simpler the better. You will use them and get good results. Simplest of all are the combination test strips that test for a few variables at once.
The first, and easiest, parameter to measure is temperature. Tank thermometers come in various forms from bulb thermometers that hang in the tank to digital thermometers that give readouts from outside the tank. There are little LCD thermometers that stick on the outside of the tank. Because these thermometers vary in accuracy, it's probably best to check them against one another. While a couple of degrees either way shouldn't make too much of a difference, some can be much more out of calibration than that.
Remember that in a small tank the water temperature will vary as the room temperature changes so take readings at different times of the day. When you change the water, make sure the water you add is the same temperature as the water in the tank. Fish vary in their needs so check what temperature is right for your tank. For most tropical communities it will be somewhere around 75 degrees Fahrenheit.
The pH is a measurement of the water's acidity. A pH of 7.0 is neutral. Above that is alkaline, below is acidic. Most tropicals do well with a pH between 6 and 7. Test strips are readily available and simple to use.
Depending upon where you live and where you get your water, it may contain more or less salt. Since saltwater is more dense than freshwater, measuring the density, called specific gravity, gives you a measure of the amount of salt. In freshwater tanks the specific gravity should be very close to 1.000. A couple of hundredths of a point more and you've got a saltwater tank. A hydrometer is used to measure the specific gravity and these are easily found in pet shops.
Ammonia, Nitrites and Nitrates
These may be the most important measurements you can take on your tank's water. The ideal reading will be zero for all of them. Getting there means taking your time setting up your tank and getting your biological filtration working. When these readings start to rise fish start to die. All of these readings are made in either milligrams per liter (mg/l) or parts per million (ppm). They reflect the balance of waste produced by the fish and the bacteria that break it down. Ammonia is the most dangerous. At 6 ppm your fish will begin to die. Controlling these three takes patience in setting up your tank, adding new fish, and making regular water changes. Test kits are extremely useful since if you see changes occurring you can take immediate action with water changes and filtration.
Again, where you live and where you get your water will dictate the hardness, or concentrations of calcium and magnesium ions. Most tropical fish like water on the softer side - 50 to 200 ppm. Hard water begins at over 200 ppm. Check the needs of the specific fishes you keep.
Oxygen and Carbon Dioxide
Since these two are part of the respiratory cycle of your tank and your fish, they ought to be watched. If your pH is good, the oxygen and carbon dioxide balance is also probably good since these dissolved gasses contribute to the overall acidity of the water. Lack of dissolved oxygen is what kills fish in polluted streams and your tank can become polluted quickly. A “DO” (dissolved oxygen) level of 7 to 10 ppm is what you should aim for. There's no need to become obsessive about taking these measurements. Once the tank is set up and running, take measurements every few days (although temperature should be kept an eye on every day). Most important is to know the demands of your fish and the limitations of your tank.