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 eventually is released back into the water as urine or feces, which also adds to the detritus.
In addition to the junk you can see, invisible waste by-products 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. Nitrate also promotes 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.
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 ten to fifteen percent of the water each week. If your tank is heavily stocked, you should bump that up to twenty 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.
Water Changing Tips
How to go about changing the water warrants an article of its own, complete with photos. In the meantime, here are a few water changing tips to consider.
- Age the water – Letting the water sit for a day will dissipate dissolved gasses that might otherwise be harmful to the fish. It will also allow the pH to stabilize before being added to the tank.
- Clean the gravel – When siphoning off the old water, vacuum the substrate at the same time. That way you get rid of some of the detritus that is building up.
- Don't touch the filter – Cleaning the gravel disturbs the beneficial bacterial colonies. The filter is the other location where these colonies grow. It's not wise to disrupt both locations at the same time. Time your filter cleaning so it occurs between water change days.