More than any other factor, water change is the most important thing in maintaining an aquarium’s water quality, and thereby, the fish’s health.
Many people believe that topping up evaporated water is water change. This is one of the most dangerous mistakes that is made in keeping fish. Evaporated water is pure water. That is how distilled water is made, by evaporating (boiling) water and collecting the steam. This process leaves all impurities and minerals behind.
Similarly, in a fish tank, evaporated water leaves behind the minerals and impurities. Then you add more tap water, concentrating those impurities. This makes the water get harder and harder as the minerals build up in the water.
The other benefit to proper water change is the removal of waste products that build up in the tank. Each time you feed the fish, after the fish use the nutrients, they expel waste, which stays in the tank. You may not see it, as the bacteria in the tank break it down, but it is there. These waste products can also change the water chemistry as well, causing the water to become increasingly acid over time.
So, if proper water changes are not done, the tank water, which may look fine and clear, gets progressively harder and more acid. Your fish get used to this gradual change, and can look healthy until they hit the crashing point. And that point can come with the introduction of a new fish. The new fish, not used to the hard acid water your fish have gradually become used to, goes into shock, is stressed, and can break out in a disease, like Ich. And then the rest of the fish, who are stressed but look healthy, come down with it as well, and you have a whole tank wipe out, which is usually blamed on the new fish, but it is actually the improper water changes and bad water that caused the problem.
Changing water doesn’t need to be difficult. 10 to 20% per week is usually more than enough, and isn’t enough to stress the fish. More than 20% at a time can be stressful on the fish, so smaller, more frequent changes are recommended (and easier for you).
When changing the water, use a “gravel vacuum” to remove the solid waste building up in your gravel. But don’t try to do the whole bottom all at once. Break it up into sections, doing about 1/4 or 1/8th of the bottom each week. That way, you can do a better job on the section, and the whole bottom gets cleaned each month. You can use tap water that has been treated with Aqua+, and is around the same temperature as the tank.
Proper water changes will help your fish stay healthy, your tank look clean and your aquarium ownership a rewarding hobby.