Whether you’re new to keeping fish or you’ve had an aquarium for a while, one of the most difficult things to determine is how often you need to change your aquarium water. 

It doesn’t help that there is so much conflicting evidence about the subject, either. Some people feel as though weekly change is absolutely necessary for the health of their fish, while others believe that an aquarium’s water only needs to be changed every few weeks.

So, who’s right? We decided to get to the bottom of this conundrum once and for all. Below, you’ll find all the information you need on how often to change aquarium water.

We’ll go in-depth into the differences between full changes and top-ups, and we’ll cover any details that you may have been confused about up until now. 

In the end, you’ll know exactly how often you should be changing the water in your aquarium, and keeping your fish healthy and happy in the process. 

How Often To Change Aquarium Water

How Often Should You Change Aquarium Water?

Let’s get straight to the point. You should change a portion of the water in your aquarium every week. The operative word here is “portion”. You don’t need to do a complete water change every week

The amount of water you should change will also differ from week to week. 

For example, you should change 10% of the water each week, 20% every two weeks, and 25% every four weeks. By doing this, you’ll be keeping the water stable and you’ll be less likely to have to do a complete water change as regularly. 

However, it’s important that you don’t just play it by eye. To keep the water clean and your fish healthy, you need to think about how much water your aquarium can hold.

So, if you have a 120-gallon aquarium, you’ll need to change 12-gallons of water every week for a 10% water change. 

With all of this in mind, here are the basics of how often you should change your aquarium water:

  • Change 10% of the water every week in line with the capacity of your aquarium
  • Change 20% of the water every two weeks in line with the capacity of your aquarium
  • Change 25% of the water every four weeks in line with the capacity of your aquarium

It’s really that simple. You can, of course, do a full water change every so often if you feel the need to. But, by changing different amounts of water over a 4-week period, you’ll keep the water both clean and stable. 

Why Should You Change Aquarium Water?

You might be wondering why it’s important to change the water in an aquarium. Once you’ve added your substrate, plants, ornaments, and placed your fish inside, your aquarium will soon become a closed ecosystem. To keep this ecosystem thriving, it’s up to you to make sure that the water is absolutely pristine at all times. 

Filtration systems do a good job of helping with this and will prevent the build-up of harmful toxins, but they have their limits.

Ultimately, your fish and other inhabitants of your aquarium solely depend on you to keep them alive and healthy. Regularly changing the water does this. 

By changing the water in your aquarium, you’re also reducing the buildup of nitrogen-based toxins including ammonia, nitrates, and nitrite.

These start to build up when too many fish are added to your aquarium or you’re overfeeding. But, when you change the water, you’re removing these toxins from your aquarium. 

You’re also given the opportunity to remove any detritus or decaying material when you change the water in your aquarium. Give the sides of the glass a wipe down with a clean sponge, and use an aquarium vacuum to disturb any settled organic matter.

Doing this before you change the water allows you to physically remove it during the water change. As a result, it won’t have the opportunity to break down into ammonia, nitrates, and nitrite.

Conducting a regular water change ensures better water clarity, too. Algae, waste particles, and airborne particles in your home can all find their way into your aquarium and these can lead to the water looking discolored and cloudy. 

This is bad for two reasons. First of all, it just makes the water in your aquarium look dirty. This blocks your view of your beautiful fish and generally makes it look very unkempt. 

Secondly, discolored water prevents light from penetrating into the aquarium. If you have live plants or corals in your aquarium, they will rely on light to photosynthesize.

If they can’t get the light they need, they’ll soon begin to weaken which leaves them more susceptible to diseases and, ultimately, can kill them. 

Water Temperature

One thing to keep in mind when you’re conducting a water change, especially if you have tropical fish, is that the temperature of the new water should be as close as possible to that of the existing water in your aquarium

Water that is straight out of the faucet will likely be much colder, so don’t put it straight in. Instead, gather as much water as you need in a container and allow it to come to room temperature before you add it to your aquarium.

It won’t be quite as warm as the water in a heated aquarium, but it will be much closer than water fresh out of your kitchen faucet!

If you’ve already emptied a percentage of your aquarium water, don’t worry too much. Your fish will be able to swim quite happily in the remaining water until you’re ready to add your room temperature water to the tank. 

Final Word

The main thing to remember here is that you absolutely don’t need to change all of the water in your aquarium every week. Likewise, you shouldn’t leave it untouched for weeks. Instead, change 10% every week, 20% every two weeks, and 25% every four weeks. 

Doing this will keep the levels in your aquarium stable, keep the water clean, and allow your filtration system to work more efficiently.