How to Change Fish Tank Water

Medically Reviewed by Amy Flowers, DVM on March 07, 2023
4 min read

To keep your fish healthy and happy throughout their life, you need to understand how and why to change your fish tank water. There are thousands of fish breeds, and each one requires different care and keeping of their tank. It is important to remember that your fish’s overall health is a direct reflection of their aquarium. Maintaining a clean tank will improve their well-being and lengthen their life. 

Before you attempt to change your fish’s water on your own, check with your veterinarian or pet store to see how often you should change it. Generally speaking, you should check pH and water levels in your tank daily. If you have a saltwater tank, it’s also important to maintain proper salinity.

Changing your fish's water regularly is important because even if your tank water looks clear, particles of food and waste are still present. Not only does this cause a physical buildup, but waste converts into chemicals such as nitrate and phosphate.

Water also needs to be changed to reintroduce elements and minerals necessary for your fish’s well-being. As time passes, elements and minerals are used up by your fish or are filtered out of the water, changing the overall pH of the water.

Steps for changing fish tank water.  Freshwater tanks are much easier to maintain than saltwater tanks. In fact, veterinarians advise against completely cleaning and replacing the water in your freshwater tank because it may shock your pet’s system. 

It’s best to keep your fish in the fish tank when you clean. Removing them causes unnecessary stress for your fish, and you run the risk of accidentally hurting them. It is possible to keep your fish in the tank while you clean because you don’t need to remove all the water to clean the tank properly. 

Completely replacing the water in the fish tank is a bad idea because it will remove beneficial bacteria that live in the tank and reset the nitrogen cycle, which could kill your fish. If you regularly clean your tank, doing a partial water change is the best option.

Keep your fish’s water clean, but not sterile. Good bacteria is necessary in the water to help maintain your fish’s health. Your tank develops a natural balance that supports your pet fish. To avoid changing the water completely, you can do it 10%-15% at a time to allow for the introduction of new water over time. 

Here are some tips for maintaining your freshwater tank: 

  • For a smaller tank, change out 10%-15% of the water each week.
  • For a larger tank, change 20% of the water each week.
  • If you use tap water, allow it to sit for three days.
  • Use a dechlorinator before pouring fresh tap water into your tank.
  • Test the water quality daily to ensure balanced pH levels.

Unplug your equipment. Don’t damage your tank filter or other systems by leaving them on while you change the water.

Gather your supplies. This should include:

  • Sponges
  • Gravel or sand siphon
  • Two clean buckets (for saltwater fish)
  • Saltwater mixture (for saltwater fish)

Clean the inside of the glass. Use a sponge to gently remove any buildup on the glass inside your fish tank. Complete this step before replacing the water.

Prepare your buckets. You’ll need one bucket for newly mixed saltwater (for saltwater fish) and another to empty existing tank water into. Make sure you don’t remove more water than you can put back in.

For saltwater fish, follow the instructions on your aquarium salt to reach the appropriate salinity level for your fish. You can mix the water in advance and store it as needed. Make sure you don’t use fresh tap water because it has additives that are dangerous for pet fish.

Remove debris. Work methodically using a sand or gravel siphon, removing debris from the bottom of your fish tank. As you work to remove the buildup, the water will become clearer. Work slowly so you don’t harm your fish in the process.

Time to repeat. By repeating this process, you can ensure that most of the debris gets cleaned up. As you siphon out debris, make sure you don’t let the tank levels go down more than 25% per water change. This ensures that your fish can easily acclimate to the new water without it shocking their system.

Refill your tank. Slowly add in the amount of water you removed while cleaning. Test the water to ensure that you reach appropriate pH and saline levels. Plug your filter in to remove any remaining debris floating around.

Here are a few other things to keep in mind to help keep your pet fish healthy.

The role of tank quality. When you purchase fish, they may be much smaller than they will be as adults. You may be tempted to get a smaller tank for less maintenance, but a larger tank will contribute to better overall health. Plus, the upkeep on a smaller tank isn’t any easier than the upkeep on a larger tank.

Regular maintenance. While you don’t have to change the water completely, each week you will have to check water levels and add water to replace what evaporates. If you have questions, talk to your local pet store or veterinarian’s office about the requirements for your specific fish breed. 

Prepping your water. Maintaining a certain pH level is important. Don’t pour water directly from your tap unless you allow it to sit for a few days because there are additives like chlorine and other compounds in the water. Alternatively, use a dechlorinator before transferring the water to your fish tank.