It is important to routinely carry out water testing to find out if the environment, which your pet is living in, is okay. If the water has extreme temperature beyond the range your tropical fish will feel comfortable, and then your pet will surely suffer. Same goes if it's too alkaline or acidic because fish will become uncomfortable. If measures are not being taken to adjust back the water condition, it can result in death. In order to ensure your tropical tank has a healthy environment suitable for your pets, make sure you test out the water on regular basis to check for problems.
First and foremost thing to do is to confirm the water temperature with a tank thermometer. Water that is too cold or too hot will leave your tropical species vulnerable to disease. It is therefore important to select the best location to place your tank or else you could risk fluctuating the water temperature. Places like air conditioning room or close to kitchen and oven should be avoided at all. The best temperature depends on what kind of tropical species you have. Usually temperature change can also happen when you add in new water. Newly purchased fish that comes in bags should leave it floating on top of the tank so that the water in the bag will be close to the tank water. Temperature shock is the most common thing that can harm newly added tropical fish.
pH is a measurement of the acidity or alkalinity of the water. These days, testing water pH can be done quickly without having to spend a ton to purchase the equipment. Some comes in pH strips, which can give accurate results just by dipping in the water. Tropical fish usually prefer pH 6 to 7 and anything out of this range can be harmful to their survival. Some of the common causes of pH fluctuation are due to ammonia buildup coming from dead fish or aquarium plants. Avoid using corals with could cause the water to turn alkaline.
Ammonia, nitrite and nitrate tests are also important. These parameters are part of nitrogen cycle and you should check the concentration individually. Ammonia and nitrite should be close to zero while nitrate at the lowest concentration. In order to achieve well-balanced environment; make sure that your tank is fully cycled. Check out my previous post on New Tank Syndrome to understand this topic much better.
Lastly, general hardness and carbonate test kits are also important. Depending on which tropical fish, some of them prefer slightly hard to soft water and don't worry if your tap water doesn't meet this requirement. There are quite a number of aquarium solutions sold in pet store, which you can easily, purchase to cure and make adjustment to the water parameter. Remember, your tropical fish is more important than anything else and nothing is more hurting to see all your efforts going down the drain just because you are ignorant and choose to ignore the water quality. Testing tank water is the key to become a successful fish owner.