How to Change Water in a Fish Tank

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Things You'll Need

  • 2 large buckets

  • Tubing

  • Water conditioners


Do not remove everything from the tank when you change the water, and do not remove all algae from surfaces and ornaments. Some kinds of fish eat some kinds of algae, and altering the composition of the water drastically can affect your fish. Do not change your water filter at the same time that you change the water. This can change the composition of the water and cause stress to your fish. When dumping water, be sure to bend your knees to prevent any back injuries.


You should change 15 to 20 percent of the water in your fish tank once a week. If you have very few fish in a large tank, you may be able to change 25 to 20 percent of the water in your tank every two weeks. Test the water in the tank frequently to be sure you are adding the necessary conditioners to the water, and to keep an eye on nitrate levels. You can plant live water plants in your tank to help keep nitrate levels down. Plants feed on nitrates, but they will not remove all nitrates from the water. Make sure your fish are not plant-eating fish before you add plants to the tank.

Keep your fish healthy by changing only part of the water in your fish tank.

One of the biggest mistakes fish owners make is changing all of the tank's water. This can be devastating as bacterial levels can be wiped out. Bacteria convert fish waste and decaying food into nitrates, which have a low toxicity. If the levels of nitrate build up in your fish tank, your fish can be affected by infections and fungus. At the same time, if you change all of the water in the tank at the same time you can alter your pets' entire environment. The best solution is dilution. Change part of the water regularly to keep your fish healthy.


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Step 1

Fill a bucket with tap water that will be used to replace the water in the fish tank. Prepare this a day in advance. The general rule is to take out about 20 percent of the tank's water. The most common tank size is 40 gallons, in which case about 8 gallons should be removed. This can be lowered to 5 gallons to make it easier, since buckets often hold 5 gallons.


Step 2

Add tap water conditioners into the bucket. The kind of conditioners you use depend on the water in your area and the levels of nitrates and ammonia in your fish tank. Test the water in the tank and from your tap with a water testing kit to be sure. You may need to use an ammonia reducer or a buffer, but it is always recommended to use a dechlorinator. Use the water conditioners and dechlorinator as directed on the container. If you have a reverse osmosis dionized (RO/DI) water filter, you will not need to use water conditioner.


Step 3

Place an empty bucket and the tubing next to the aquarium.

Step 4

Turn off all electric devices used in the fish tank to avoid any problems. Heaters can break if exposed to excessive air, as they will attempt to heat to the room's temperature. Filters can go dry if water levels go below the suction tubing.


Step 5

Place one end of the tubing into the fish tank and initiate suction by sucking on the other end of the tube like you would a straw. Make sure your end is below the end in the tank so that the water will be pulled down into the bucket. Be sure to watch the water move down the tubing to avoid getting any in your mouth. Before the water gets to your mouth, place the tubing in the empty bucket.


Step 6

Remove any solid waste inside the fish tank by gently siphoning the gravel with the tubing to pick up any settled debris. Use the end of the tubing inside the tank to do this.

Step 7

When the bucket is almost full, remove the tubing from the fish tank.


Step 8

Pour the prepared water into the tank.

Step 9

Plug in any electrical equipment that was unplugged in Step 4.