How Often Should You Change Your Fish Tank Water?

Water changes are essential for a healthy aquarium but how often should they be done? We have all the info right here.

May 18, 2024By Kenny Jarvis
how often should you change your fish tank water

Being a fish owner is about much more than just adding fish to a tank. Aquariums need regular maintenance in order to give them a healthy environment. Without one, your fish will get sick and eventually die.

One of the key parts of fish tank maintenance is regular water changes. The frequency of these changes can hugely affect their environment, so how often should it be done? Here we’ll answer that question while also giving you all the info you need about water changes.

How Often Should I Do Water Changes?

water change
Source: NT Labs

There is no exact rule when it comes to water changes. The volume and frequency of your changes will depend on several factors that we’ll explore below. That being said, there is a general rule to follow.

If you aim to change 20% of the water every two weeks, then you won’t go far wrong. After these changes, you can then check the water parameters and the behavior of your fish. You’ll eventually get to a point where you’ll know what’s best for your tank.

While water changes are vital, it's important to know it's not an exact science, and there are differing views on how often they should be done. As long as you change a significant portion of the water often, you can have peace of mind.

Why Water Changes Are Vital?

change aquarium water
Source: Sea Tech H2O

Water changes are something a few beginner tank owners don't even consider. After all, there is no such thing as water changes in the wild, and, as humans, we don't need air changes. However, the difference is that your fish tank is a closed environment. Here's why you should be changing the water.

Removal of Toxins – Fish waste and uneaten food will break down into ammonia, nitrites, and nitrates. These substances are toxic to fish, and without a water change, they will eventually become deadly. While changing water, it's a great idea to remove this debris from the bottom of the tank.

Maintaining pH and Mineral Levels – Plants, gravel, driftwood, and other factors can change the pH in your tank. Also, essential minerals will be depleted over time. Adding freshwater helps to restore the pH to more neutral levels and replenish minerals.

Oxygenation – Like us, fish need oxygen to breath, except they do it through their gills. Oxygen depletes in a tank over time, and without fresh water or oxygen absorption, your fish would eventually suffocate.

Factors Influencing Water Change Frequency

water change small tank
Source: YouTube

All tanks are different and the frequency of water changes required for one tank owner may be different from another. To better understand how often you should be changing your water, let’s look at the factors involved.

Tank Size – The smaller the tank, the more quickly toxins can accumulate. With small tanks, it's best to go on the side of smaller but more frequent water changes. Larger tanks can be left for a longer time and change a higher percentage of the water when you do it.

Number and Size of Fish – This is a simple equation of more fish equals more waste. A densely stocked tank is going to need more frequent water changes. You’ll also need to be more vigilant with water testing to ensure toxins aren’t spiking.

Type of Fish – Some fish produce more waste than others, commonly referred to as a bio load. Fish with high bio loads will need their water changed more often. Check to see if the fish you keep produce a lot of waste or not.

live plant aquarium
Source: Glass Grown Aquatics

Filtration System – An efficient filtration system can help to take toxins out of the water and help it absorb more oxygen. The better the filter, the less frequent water changes need to be. It’s important to note that even with the best filter in the world, water changes are still needed.

Plants – Live plants can help to absorb some of the harmful toxins while also adding oxygen to the environment. However, plants can be high maintenance, so if you’re hoping to enjoy a low maintenance tank, they aren’t suitable for you.

Feeding Habits – New fish owners have a habit of overfeeding fish as they are worried about not giving them enough food. This can lead to excess waste buildup in the tank. Double-check you're giving your fish the right amount of food.

Water Testing – You should regularly be testing your water and this can easily be done with cheap testing strips. If you get alarming results, then you will need to change your water. If you have only recently changed the water, then you need to investigate the problem.

Guidelines for Water Change Frequency

large water change
Source: Reddit

There, we mentioned that changing your water every two weeks is a common method, but it can be longer or shorter than that. Here are some general guidelines to ensure a well-balanced aquarium.

Weekly Partial Water Changes – For smaller tanks with a high bio load, more frequent changes may be needed. Here you’ll want to change around 10% of the water but up to 25% is fine. This will help to keep your toxins in check.

Bi-Weekly Partial Water Changes – With a lightly stocked small tank or a medium-sized aquarium, bi-weekly water changes are usually the best. Changing around 20% to 30% of the water is a good figure. Remember, this doesn’t need to be exact, and you can just eyeball it.

Monthly Water Changes – For heavily planted aquariums that aren’t overstocked, monthly changes can also work. You’ll want to keep checking your water parameters and a water change of up to 50% can be done here.

Emergency Water Changes – If in doubt, add fresh water. If you see any signs of distress, such as gasping for air, immediately change the water. This is regardless of when your last water change was done.

Tips for Correct Water Changes

water change siphon
Source: Sea Tech H2O

The frustrating thing for new fish owners is knowing that fresh water can kill your fish. Why? Well, because the water is likely to contain chemicals and be at a different temperature to your tank. Thankfully, these issues can be easily resolved. Here are some tips:

Use a Siphon – Use a siphon for water changes. This not only removes the water but has the double benefit of removing waste from your substrate, slowing down the production of toxins.

Condition Tap Water – Tap water is fine to use as long as you condition it, particularly the chlorine. Chlorine is added in tap water to kill bacteria, but it can also harm your fish. A conditioner will neutralize the chlorine.

Match Temperature and pH – Make sure the new water matches the temperature of what’s in the tank or you could cold shock your fish. Also, if your tank has a unique pH, make sure it matches. This pH is rarely an issue for most fish as tap water will have a pH close to neutral.

Clean and Test – During water changes, it’s a good idea to test your water parameters and clean the tank and filter. This will prevent the risk of a sharp spike in toxins and will give you information on whether or not your water changes are being made too soon or too late.

Kenny Jarvis
By Kenny Jarvis

Kenny is a passionate animal lover who finds joy in the diverse world of pets. He frequently embarks on zoo adventures with his children, immersing himself in the wonders of nature. At home, Kenny tends to a much-loved aquarium, nurturing a thriving underwater ecosystem. Through his passion for writing, he aims to help fellow animal lovers create happy and healthy environments for their pets.