Even when your aquarium water looks clear, if you stir up the substrate a bit, you may be shocked at how much detritus is present. When fish are fed, particles of food fall to the 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 problem of poor water quality. Eventually, these wastes accumulate in the aquarium, both as solid debris in the gravel and as dissolved chemicals such as nitrate and phosphate.
And yet, waste products are not the only reason water needs to be changed. Trace elements and minerals in the water are important both to the stability of the water chemistry and to your fish. Over time, trace elements are either used up or filtered out; if they are not replaced by water changes, the pH of the water will drop. Furthermore, a lack of trace minerals will adversely affect the vigor and health of the fish as well as the biofilter bacteria that remove ammonia from the water. Giving your fish new water regularly is much the same as giving them vitamins and minerals to keep them strong and healthy.
Nitrate and Phosphate Waste
In addition to the debris you can see, other invisible natural waste byproducts called nitrate and phosphate will build up. These put chronic stress on the fish, making them more vulnerable to disease. Elevated nitrate also stunts the growth of young fish and interferes with normal reproduction in adult fish. Nitrate and phosphate also promote the overgrowth of algae; having the effect of fertilizers. Changing the water is the best way to keep nitrate and phosphate levels low.
Frequency of Water Changes
Water changes should be part of regular aquarium maintenance, but the frequency can vary somewhat, depending on size of aquarium and the number of fish. Smaller, heavily stocked tanks will require more frequent water changes than larger, sparsely stocked aquariums.
A good rule 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 aquarium can maybe get by for two to four weeks, but this should be the maximum length of time between water changes.
Topping Off for Evaporation
You might think that adding water to the tank is the same thing as changing the water, but that is not the case. Merely adding water does not remove any of the wastes, so do not skimp on the water changes. Simply topping off the aquarium water as it evaporates puts fish at risk of poor health.When you see the water level has dropped, go ahead and use a gravel vacuum to clean the aquarium gravel and remove more water, then add fresh, dechlorinated water to the aquarium to bring it back to the proper level.