Is aquarium water testing really necessary? In a word - yes!
In a newly set up aquarium, water testing is critical to avoid fish loss as ammonia and nitrite rapidly rise. In an established aquarium, water testing is important to ensure the continued health of your fish.
Test kits should be considered an important part of the operating expense associated with keeping an aquarium. If you cannot afford test kits or feel uncomfortable testing water yourself, check with your fish shop to see what they charge for doing water tests. Some offer free water testing, or at least one free test each month, or will quote you a flat fee for monthly testing. Compare their charges against the actual cost of test kits.
Ammonia, pH, nitrite, and nitrate water test kits are by far the most integral to aquarium water upkeep. Hardness and alkalinity tests are useful to establish what your levels are, but don't warrant purchasing an entire kit for them unless you have special needs such as a planted tank. Phosphate is worth testing for if you have algae problems. All testing should be recorded in a log or journal so that you have a record of what is happening over time.
Ammonia will be elevated during the start-up cycle in a new tank, but can also be elevated in mature tanks if the water is not changed regularly, filters are not kept clean, if the tank is overstocked or overfed, or if medication is used that disrupts the biological cycle.
In an established aquarium, an ammonia test should be performed and recorded in a log once a month. Anytime you have sick fish, or a fish death, you should immediately test for ammonia. Any detectable amount of ammonia should be addressed swiftly, as it is extremely toxic to fish.
The acid-base balance of the water, measured as pH, is the most frequent cause of fish stress, which can ultimately lead to fish loss. It is usually the most overlooked parameter. Fish cannot tolerate sudden changes in pH; even a change of 0.2 can result in stress or death if it occurs suddenly.
Know the pH of your fish shop's water, as well as your own, so you can help acclimate the new fish properly. Keep in mind that if you use tap water, it has dissolved gasses as a result of being under pressure. Let the tap water sit overnight before testing the pH.
The pH can, and will, change with time. Fish and plant waste, water evaporation, topping off the water, and water hardness will all contribute to changes in the pH. As a rule of thumb, pH in an established tank should be tested once a month, and any time there is a fish death or illness.
Another factor that affects the pH is the buffering capability of your water. As aquariums age, the biological filter uses up the carbonate in the water, and the aquarium will lose its buffering capacity, resulting in rapid pH changes. If your water pH changes suddenly or drifts regularly over time, you should check the KH (Carbonate Hardness, or Alkalinity) of the water. Consult your local fish shop for KH testing, and for buffering compounds to increase the water alkalinity and stabilize the pH level.
During the startup of a new tank, nitrite levels will soar and can stress or kill fish. However, even after aquarium water is initially "cycled," it is not unusual to go through mini-cycles from time to time. For that reason, include nitrite testing as part of your monthly testing routine. Any elevation in nitrite level is a red flag that indicates a problem brewing in the tank. If a fish is ill or dies, it's wise to test for nitrite to ensure it is not contributing to the problem. The only way to reduce elevated nitrate levels quickly is via water changes. Adding low levels of salt to the water - 1-3 teaspoons per gallon - will reduce the effects of nitrite toxicity.
Although nitrate is not as toxic as ammonia or nitrite, it must be monitored to avoid stressing the fish. Nitrate can also be a source of algae problems. Nitrate will rise over time and can only be eliminated via water changes. Monthly tests are important, particularly when breeding fish, as young fish are more sensitive to nitrate than adult fish. Test monthly and keep levels low to ensure a healthy tank.
Whenever anyone complains that they cannot win the battle against algae, high phosphate immediately comes to mind. Phosphate serves as a nutrient for algae, and elevated levels will certainly add to your algae woes.
Although it's rarely discussed, a leading cause of increased phosphate is dry fish food - particularly overfeeding with lower quality foods that are high in phosphate. If you have algae overgrowth, test for phosphate. There are filtering materials available that remove phosphate from the water, and they can be added into your filter media.
These are the most important water parameters that need to be tested: ammonia, nitrite, nitrate, pH, hardness, alkalinity and possibly phosphate. But don't forget to check another important water parameter, the water temperature, as this is important to keep in the normal range for the species of fish in your aquarium! A digital or floating glass aquarium thermometer is an important part of your water testing equipment.