How Often Should You Change Aquarium Water?

Regular water changes keep your fish healthy, happy and colorful.
i fish tank image by Jaimie Duplass from

Your dog and cat can escape their wastes by going outside to potty or using a litter box. Your fish friends have no choice but to swim in their wastes until you take action. Weekly water changes help, but sick or stressed fish signal a need for more frequent attention.

Tank Setup

Ensuring that your tank is cycled is vital to the health of your fish and your regularly scheduled water changes. A cycled tank is a mature tank. It houses an established colony of nitrifying bacteria, which turn ammonia and nitrite into nitrate. You control the nitrate level with weekly water changes. Your fish friends can become sick, and even die, if ammonia and nitrite levels are not kept at 0 parts per million and if nitrate levels rise much above 40 parts per million.

One of the simplest ways of cycling your tank is to use a bacterial additive, following the manufacturer's directions. The additive contains the necessary beneficial bacteria, and normally will cycle your tank within a day or two. The next simplest method is to take some gravel and filter media from an established tank and add them to the new tank. You then can begin adding fish to the tank immediately.

Water Tests

Water tests are important for two reasons: They tell you when your tank is cycled, and they let you know if ammonia, nitrite or nitrate levels have spiked and you need to make an emergency water change. If you have a healthy established tank, you only need to test the condition of your water if your fish friends show signs of stress or are becoming sick. You can check your water monthly, but your fish will let you know almost immediately when the water has become unhealthy. This can happen if you are overfeeding your fish, if a fish that has died has not been removed, or if you have not maintained your weekly partial water-change schedule. Pet stores sell strip tests and liquid tests. Both usually come with test tubes to fill with aquarium water, then dip in the strip or add the test solutions.

Standard Water Changes

Weekly replacement of 10 to 20 percent of the tank water will keep your fish happy and healthy, provided they're swimming in water that's free of ammonia and nitrite and that contains less than 40 parts per million nitrate. The water change keeps the nitrate level low and your substrate free of waste and dirt. An aquarium vacuum works best to accomplish this -- just make sure you don't accidentally suck up one of your fish buddies. You don't need to vacuum around plants, because the plants use the nutrients from the fish waste, and their roots don't enjoy being disturbed.

Emergency Water Changes

If the test shows the presence of ammonia or nitrite or excessive nitrate levels, you'll need to change the water daily, and maybe more than once per day, until the levels are lowered. Suppose your fish are swimming in 2 parts per million ammonia -- a dangerous level. Changing 50 percent of your water, followed immediately by 50 percent again, will reduce ammonia to one-half part per million, which is more manageable, but still above zero. Now you can do daily water changes of 40 to 50 percent until the ammonia level is reduced to zero with the help of the bacteria. Once your ammonia, nitrite and nitrate levels are back where they belong, you can resume weekly 10 to 20 percent water changes.

Hospital Tank

If one of your fish is ill, help him recover by medicating him in a hospital tank, separating him from his buddies so that you're not medicating your healthy fish along with the sick one. Some people believe it's important to change the water of a sick fish daily, but doing that may render the medication ineffective. Read the instructions for the medicine before you change the water.

Replacement Water

Fish need good water, not just any water. If you fill your tank straight from your tap, you may cause a spike of ammonia, nitrite or nitrate. It's best to fill jugs with the fresh water, then treat each jug with an aquarium water conditioner. These conditioners remove chlorine, chloramine and ammonia, and convert nitrite and nitrate into a form that's not harmful to fish and that the beneficial bacteria in the filter and tank can remove. Some conditioners also detoxify heavy metals. Conditioners help protect your fish by enhancing their slime coats. If you buy bottled spring water or distilled water, add conditioner just to be safe.

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