Every fish has a preferred water chemistry that mimics his home waters. Some species can only survive in a narrow range of water parameters, while others can thrive across a broader range of conditions. Water testing ensures that you have healthy, ideal conditions for your fish, and that waste products have not built up to dangerous levels.
What to Test
Most pet shops sell a plethora of test kits. Fortunately, you probably don't need most of them, unless you plan on keeping very specialized types of aquariums such as saltwater reef tanks. For most freshwater aquariums, you only need to look at pH, ammonia, nitrite and nitrate. If you plan on keeping plants or breeding rare and expensive fish, you may need additional tests for things such as carbon dioxide, hardness or alkalinity. However, pH can tell you quite a bit of these parameters. For example, water with a high pH typically has high alkalinity and hardness. And if the pH increases when the lights turn on, you could have carbon dioxide issues. If you have sensitive fish or plants and see these signs, you may need to invest in more test kits. Research the species you plan on keeping to figure out if you will need these test kits.
Testing Options: Pros and Cons
You have two main types of testing kits: reagents and probes. Reagents consist of liquid, powder or test strips that you expose to a sample of aquarium water and compare to a color chart. Probes consist of a piece of electronic equipment you stick into aquarium water that measures water parameters. Reagents have the advantage of costing less and being widely available. However, reagents expire and can be tricky to read. Probes cost much more and require calibration, but produce more accurate results and are usually very easy to read. For most people, reagents' much lower cost makes them a better solution, even with their flaws.
How Often to Test
Your testing schedule will evolve with the aquarium. When you first establish the aquarium -- a process called cycling -- test the water daily. Cut back to every third day if everything's working out fine. It typically takes up to six weeks for an aquarium to fully cycle. When your aquarium is established and you have all of your fish, you can cut back even further to weekly or monthly water testing. However, if you see signs of trouble, you should always test your water, regardless of your schedule.
Signs of Trouble
If water has too much ammonia or nitrite, fish exhibit signs of stress. Both of these compounds burn the gill tissues. Since fish use their gills for breathing and excreting ammonia, damage to the gills compounds the issues by making it harder for fish to excrete ammonia. So test your water if you see symptoms such as fish flapping their gills faster than usual or gasping at the water's surface. Additionally, if you see fading color, erratic swimming or skittish behavior, test the water to see if ammonia poisoning is causing these symptoms. Levels of ammonia and nitrite should be zero; anything higher requires immediate action. Also, whenever a fish dies, test the water. Fish may die from natural causes, but high levels of ammonia or nitrite could also be the cause.