Even if your water looks clear, stir up the substrate a bit and you'll be shocked at how much detritus (aka junk) is there. Where did it come from? When fish are fed, particles of food fall to the tank bottom where they decay. Meanwhile, the food that is eaten is eventually released back into the water as urine or feces, which also adds to the debris.
Nitrate and Phosphate Waste
In addition to the junk you can see, invisible waste byproducts build up in the form of nitrates and phosphates. This puts stress on the fish, making them vulnerable to disease. Elevated nitrates will stunt the growth of young fish and can interfere with normal reproduction in adult fish. Nitrates also promote overgrowth of algae. Phosphates have a similar effect. Changing the water is the best way to keep nitrate and phosphate levels low.
Wastes aren't the only reason water needs to be changed. Trace elements and minerals in the water are important to the health of your fish as well as the stability of the water chemistry. Over time they are used up or filtered out. If they aren't replaced, the pH of the water will drop. Furthermore, the lack of trace minerals will adversely affect the vigor and health of the fish. Giving your fish fresh water regularly is much the same as giving your kids vitamins to keep them strong and healthy.
How Often You Should Change Aquarium Water
Water changes should be part of regular aquarium maintenance. The frequency will vary somewhat, depending on many factors. Smaller, heavily stocked tanks will require more frequent water changes than larger, sparsely stocked aquariums.
My recommendation is to change 10 to 15 percent of the water each week. If your tank is heavily stocked, bump that up to 20 percent each week. A lightly stocked tank can get by for two weeks, but that should be the maximum length of time between water changes.
What about topping off? Some people think that if they add water to the tank, it's the same thing as changing the water. That is not the case. Adding water doesn't remove any of the wastes, so don't skimp on the water changes simply because you top off the tank now and then.