What is a partial water change and why your fish tank needs one

One essential aspect of aquarium upkeep is conducting regular partial water changes. You probably have heard of it, and its self-descriptive term might also give you a good idea of what it entails. But do you know why it is necessary and how to do it? We will learn that in this article.

gold fish in a fish tank
Photo by GollyGforce, cc

Partial water changes involve removing part of the aquarium water and replacing it with fresh, treated water. Usually, it is about one-third of the fish tank’s volume, but let’s dive deeper. Let’s explore why partial water changes are necessary and how they contribute to a healthy aquatic environment.

Importance of water quality

Water quality plays a vital role in the health and well-being of your aquarium’s inhabitants. Poor water quality can lead to stress, disease, and even death for your fish.

Various factors, such as fish waste, uneaten food, and decaying plant matter, can negatively impact water quality. Regular partial water changes help maintain optimal water conditions, promotes beneficial bacteria growth, and ensure a healthier environment for your aquatic pets.

Reasons for regular partial water changes

  • Removing harmful waste products. Fish produce waste (a.k.a., they poop) and release ammonia into the water. Ammonia is toxic to fish, but beneficial bacteria can convert it into nitrates. Unfortunately, it takes time, and the amount of ammonia in an aquarium may accumulate over time. Moreover, while nitrate is less toxic than ammonia, high levels can still harm your fish. Partial water changes help dilute the concentration of ammonia, nitrates, and other harmful substances, reducing the risk of illness and stress in your aquarium.
  • Replenishing essential minerals. Aquatic creatures rely on minerals in the water for various biological processes, including osmoregulation and maintaining proper pH levels. Over time, these minerals can become depleted as fish utilize them. Partial water changes help replenish essential minerals, ensuring your fish have access to the necessary nutrients for their well-being.
  • Maintaining stable water chemistry. As waste products accumulate, the water chemistry in your aquarium can become imbalanced, leading to fluctuations in pH, alkalinity, and hardness. Regular partial water changes help maintain stable water chemistry, providing a consistent environment.
  • Discouraging algae growth. Excess nutrients, such as nitrate and phosphate, can fuel unwanted algae growth in your aquarium. By performing partial water changes, you can help control nutrient levels and minimize the risk of algae blooms, which can negatively impact the appearance and health of your aquarium.
  • Enhancing water clarity. Over time, organic matter and debris can cause your aquarium water to become cloudy or discolored. Partial water changes can help improve water clarity by removing suspended particles and discolored water, ensuring a more visually appealing aquarium.

How often should you perform partial water changes?

The frequency of partial water changes depends on various factors, such as the size of your aquarium, the number of fish, and the efficiency of your filtration system. Here are some general guidelines to help you determine the appropriate water change schedule for your aquarium:

  • Small aquariums (under 30 gallons): Small aquariums have a lower water volume, which means waste products can build up more quickly. Performing partial water changes of 15-20% every week is recommended in this case.
  • Medium to large aquariums (30-100 gallons): For larger aquariums, you can perform partial water changes of 20-30% every two weeks. This schedule helps maintain water quality without causing significant fluctuations in water parameters.
  • Heavily stocked aquariums: If your aquarium is heavily stocked with fish, you may need more frequent water changes to manage the increased waste production. In such cases, 20-30% weekly water changes may be necessary.
  • Lightly stocked or planted aquariums: If your aquarium is lightly stocked or contains many live plants that help absorb waste products, you may be able to perform partial water changes less frequently. A 20-30% water change every 2-3 weeks should suffice.
  • Marine and reef aquariums: Saltwater aquariums, particularly those with sensitive coral and invertebrates, may require more frequent and smaller water changes to maintain stable water chemistry. A weekly water change of 10-15% is typically recommended for these systems.

It’s essential to regularly monitor your aquarium’s water parameters using test kits for ammonia, nitrite, nitrate, and pH levels. If they exceed recommended units, you may want to increase the frequency of water changes or increase the proportion you change each time. This will help you determine if your current water change schedule is sufficient or if adjustments are needed. Learn about the most important aquarium water parameters here.

Remember, every aquarium is unique, and the optimal water change frequency may vary depending on your specific setup. Pay close attention to your fish’s behavior and the overall health of your aquarium to help guide your maintenance schedule.

How to perform a partial water change

To perform a partial water change, follow these easy steps:

  1. Turn off aquarium equipment, such as heaters and filters, to avoid harm during the water change process.
  2. Use a gravel vacuum or siphon to remove approximately 20-30% of the aquarium water while simultaneously cleaning the substrate.
  3. Prepare fresh, treated water using a water conditioner to remove chlorine and other harmful substances. You can also achieve this by letting the water sit in a container for a day or two. Ensure the water temperature is similar to the aquarium’s to avoid temperature shock for your fish.
  4. Slowly add the fresh water to the aquarium, taking care not to disturb the fish or decorations. You can pour it over a plate or your palm to reduce the force of the jet.
  5. Turn the aquarium equipment back on and monitor the water parameters to ensure they are within the appropriate range for your fish.


Regular partial water changes are essential to aquarium maintenance and contribute to a healthier and more stable environment for your fish and other aquatic life.
By removing harmful waste products, replenishing essential minerals, maintaining stable water chemistry, and enhancing water clarity, you can ensure your aquarium remains a thriving, visually appealing habitat for your aquatic pets.





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