How Often To Change Water in Fish Tanks | Expert’s Strategy

Changing the water in a fish tank can be a tricky task. Fish wellbeing largely depends on water change. Therefore, It’s essential to know how often to change the water in fish tanks and how to do that correctly to prevent unwanted fish death.

As a general rule, 10-25% of the water should be changed weekly. However, the frequency varies on stocking density, types of fish, filtration system, live plants, and whether the nitrogen cycle is established or it’s a new tank. Also, some tanks don’t require a water change at all.

For instance, if there are more than 50 gallons of water in a tank with no filter, then it will need to be changed every 2-3 days. However, if there is an efficient filter system with less than 10 gallons of water for one small betta fish, then you only have to change the water once per month!

Testing water quality is crucial to understand water test frequency
Testing water quality is crucial to understand water test frequency

So, how often should you change the water for your tank? Luckily, there are ways to identify accurately how often to change the water in a fish tank. In the below section, I’ll let you know.

  • How often should you change aquarium water?
  • How much water should you change?
  • How often to change the water in a fish tank without a filter?

Why Should You Change The Water In Your Aquarium? 

Fish are cute and cuddly, but they are not habituated to living in their own waste. The only way to keep them healthy in aquariums is by using a biological filter to break down their waste or changing the aquarium’s water regularly. 

Without using biological filtration or regular water changes, these creatures would die from being poisoned by their own bodily fluids! So, let’s know why changing water is so crucial. 

#4: Prevent Detritus Build-Up & Waste Accumulation

Fish eat and like all animals they poop! Moreover, uneaten food particle sinks to the bottom and decay, which also adds up to the problem of poor water quality. 

The accumulation of waste in an aquarium is a never-ending cycle. Wastes accumulated both as solid debris and dissolved chemicals, such as nitrate or phosphate. Those are harmful to fish living there if it’s not monitored properly. 

Once you cycle your fish tank initially, beneficial bacteria colonies grow in filters, substrates, and all over the aquarium space. They convert toxic ammonia excreted by fish into nitrite (also harmful), and then into nitrate (less toxic). 

Sponge filter with air tubing
Sponge filter with air tubing

Nitrate constantly builds up in your aquarium (a byproduct of biological filtration). The higher your nitrate levels, the less likely it is that you’ll have a healthy and productive tank. The reason is that elevated nitrates hinder growth in young fish and reproduction in adult ones. 

You need to change the water also to lower phosphate levels, remove tannins, and maintain optimum pH and KH. High Nitrates and phosphates are bad for fish. They cause chronic stress, which makes them more susceptible to disease, as well as promotes the overgrowth of algae in the water.

If you don’t change the water frequently enough, over time, the nitrate level will likely rise, and pH and KH will dip too low, which can ultimately lead to Old Tank Syndrome.

A biologically effective filter system stops working when the pH level drops so low, and fish will die within hours or days in this old tank environment. 

Why Should You Change Water Regularly
Why Should You Change The Water In Your Aquarium

#2: Replenishing Minerals & Trace Elements

Reducing waste accumulation is not the only reason for a water change, but it’s worth noting that there are other reasons too. For example, you shouldn’t let your fish suffer from a lack of minerals in their tank environment!

You know that fish need vitamins and minerals to stay healthy, right? Well, new water is just as crucial for them. So, if you want your pet fish to be strong and vigor, then make sure they’re getting their appropriate dose of trace elements by replenishing them at specific intervals!

Over time trace elements and minerals are used up or filtered out. Those are crucial for the stability of water chemistry and for your fish.

While these important nutrients also help maintain the pH level, they also supply vital functions for the well-being of the aquatic critters in an aquarium environment. 

How Often To Change Water In Fish Tank? 

How often should you change the aquarium water? Every tank is different, so it’s important to know and understand your specific bio-load, and the tank setup you have. This will allow for a more individualized schedule on the water changes!

Survey Results shown in a bar chart
Survey Results shown in a bar chart

I conduct a survey questioning how often should I change the water in my fish tank. I have got responses from over 300 seasoned aquarists who have been keeping fish, breeding, raising fries, and maintaining fish tanks for many years.

The result may surprise you a little. In summary, among the fishkeepers, 44% change water weekly as a routine, 14% do that after testing the water, and 12% never change the water, they only top off when it requires.

In the below table, you’ll find the top results from the survey.

How Frequent Fishkeepers Change Water% Fishkeepers
As needed, After testing the Water14%
Never, top off only12%
Not that simple3%
Every tank is different 3%
When visibly dirty1%

So, as you see, a total of 6% think water change is complicated, it’s not that simple and the frequency will vary from tank to tank since every tank is different. A good percentage of people never change the water. On the other hand, others change the water on a routine basis, either weekly, bi-weekly or monthly.

Another approach is to test the water before a water change. So, if you want to change the water after testing, you need to figure out how much waste your fish produces. You can do it by measuring the nitrates present in the water.

So, how can you set the water change frequency based on the nitrate level? There’s an easy way.

Your target should be to keep the nitrate level below 40 ppm at any given time. Suppose you have an aquarium that generates 5 ppm of nitrates every week. So, it takes eight weeks to reach the threshold limit. 

How often to change water in aquarium

If the nitrates level reaches 40 ppm and you change 25% of water, you reduce nitrates by 25%. Meaning after a water change, nitrates will be 30 ppm.

After two weeks, nitrates will climb back to 40 ppm. So, with the current trend, changing 25% of the water every two weeks will keep the nitrates level below 40 ppm. 

However, if anything changes, like if you overfeed than usual or add more fish, you should look for the rate of nitrates change and adjust your water change frequency. 

Tips: Sometimes tap water contains some nitrates (usually below 10 ppm); test the tap water too before adding it to the aquarium because it can hamper the calculations. If the tap water has nitrates, use RO water or rainwater, which is purified and free of nitrates. 

Factors That Influence Water Change Frequency

Simply asking the question of how often you should change the water is not the right approach. Instead, you should provide some additional context to get your answer, because it’s subjective. It depends on many factors. So, let’s figure those out.


Bio-load of the aquarium is vital for determining water change frequency. It depends on the fish density, types of fish, and aquarium size.

It’s evident that an aquarium containing a large number of fish will have more bio-load compared to a sparingly stocked fish tank. Some fish relatively generate more waste, such as goldfish, than the other fish species.

On the other hand, a few bottom-dwelling fish species help manage the bio-load by consuming leftover foods, and other waste at the tank bottom.

If you have a bigger tank the waste will take time to climb up compared to a smaller tank.

Factors That Influence Water Change Frequency

Filtration System

The filtration system is always important. If you have a strong enough filter which can manage the bio-load then you will need less water change.

Therefore, it’s recommended to buy a bigger filter so that you can get the advantage.

Live Plants

Live plants are extremely crucial when it comes to water change. The more live plants you have the less you have to change the water.

Live plants help in the filtration of the tank. They help to oxygenate the tank and maintain a healthy substrate. They also provide food for the fish, emulate the natural habitat, and help in algae reduction.

So, if you have a heavily planted aquarium you don’t need to change the water much. Fishkeepers who never change the water maintain a heavily planted aquarium.

In addition, right fish density, and the presence of other micro-critters help them to establish complete harmony. It’s like achieving a self-sustaining ecosystem in the fish tank.

As a piece of extra information, those who never change water need mineral supplements like calcium and other minor components to maintain mineral balance.

New or Established tank

The frequency of water change depends largely also on whether the tank has an established nitrogen cycle or it’s a new tank.

If you just set up your tank when beneficial bacteria colonies have not developed yet, do a weekly change till a few months after your tank cycled.

You want to make sure tank parameter levels are stable for a few weeks before you can revise the water change frequency.

Topping Off Vs. Water Change

The water level in the aquarium drops gradually due to evaporation. So, you may think that the addition of the lost water is the same as the water change. However, merely adding water is not enough to remove the wastes from your tank, so don’t skimp on the changes! 

Because by adding water, you are just diluting the wastes but not removing them. If you don’t remove debris through a water change, waste will continue to climb up and eventually affect the fish. 

A reduced water level is also a good indication that it’s time for a water change. When you see the water level has dropped, go ahead and use a gravel vacuum to clean out the substrates and remove a bit more water.

Then add fresh dechlorinated water (or distilled if possible) until it’s back at an acceptable height for proper fish health and happiness!

Tips: When it comes time for your fish tank to be refilled, make sure you use a clean and safe water source. Age the water one day, and make sure the temperature and pH of the water are close to the ideal range for your fish. 

How Often To Change The Water In A Fish Tank Without A Filter

Toxic materials will climb up fast in fish tanks without filters because there are no ways to remove wastes out of the aquarium. As a rule of thumb, change 10-15% of the water daily in tanks having no filter. However, you can also check the nitrate increase rates and determine the water change frequency.

A daily water change will not only remove detrimental chemicals that can decrease water quality but also help reduce debris like uneaten fish food. However, if you do leave it behind, this too could contribute to an adverse effect on the aquarium environment. 

How Much Water To Change At A Time? 

You already know that the frequency of water changes for an aquarium depends on the aquarium size, number of fish, rate of waste accumulation, and so on. Usually, heavily stocked smaller tanks require more frequent water change than larger ones with less fish density. 

Change 10-15% of water if you do it on a weekly basis. If you have tanks of high fish density with no live plants, bump that up all the way to 25% weekly. But, if you can’t do that every week, change 20-25% bi-weekly.

If you have live plants in large quantities, weekly or bi-weekly 10% water change will be sufficient enough. Some planted tank owners extend the changing time even further. For light fish populations, two to four weeks should be long enough before it needs another change over again!

Aquarium Types Frequency & Change Amount
Average Aquarium 10-25%, Weekly or 20-25%, Bi-Weekly
Heavily Stocked 20-25%, Weekly
Lightly Stocked 10-15%, Bi-weekly
Heavily stocked, Heavily planted 10% weekly or bi-weekly
Heavily stocked, Lightly planted 15% weekly or 25% bi-weekly
% water change for types of aquarium

Limit the water change to a maximum of 30% of the total volume at a time. The goal of fishkeepers is to keep their aquatic friends happy and healthy. With that in mind, drastic water changes can be very stressful for them.

So it’s crucial not only to recognize how much work goes into doing large-scale changes but also to think about whether or not these actions are necessary at all.

Tips: When performing water changes, make sure to use an aquarium gravel vacuum and bury the head in the substrate. This ensures that any toxins or accumulated debris are also removed along with the water!

Tips To Decrease The Water Change Frequency

You may feel it’s overwhelming to change the water every week, and probably you’re finding ways to reduce the frequency. Below I’ll share some tips which you can apply to decrease the water change frequency. 

Tips To Decrease The Water Change Frequency
  • Buy a quality filter because, without a filter, you have no control over the waste. Therefore, a quality aquarium filter is a must-have if you want to avoid water changes very frequently. 
  • You can help yourself by making sure that you are not overfeeding and overstocking. You should also feed less or simply keep fewer fish to balance out the tank environment.
  • Increase the water volume because more water with the same number of fish will spread the waste over the water body. In other words, buy a larger aquarium instead of a smaller one at the beginning. 
  • Plants take nitrates as nutrients; therefore, adding live plants that are good at reducing nitrate will help to some extent to keep the nitrate levels low. But please don’t get me wrong, live plants will not cancel out the requirement of water change. It can only be a great addition to reducing the change frequency. 

How To Do A Water Change? 

First, mark the water level in the fish tank to the extent you want to remove it. After that, to do a water change, you will need: 

  • A siphon tube and bucket to start with-The siphon should be started by hand, not sucked for hygiene reasons! Place one end of the tube in the tank and the other end in the bucket; start the siphon, and you are on the go! Keep close eyes on the water level, and no small fish are sucked up in the tube during the process.
Siphoning a fish tank
  • Aged water – Throw out the dirty water to the sink, or use it for other purposes like watering the garden plants (as plants may suck nutrients from the fish waste). Refill the tank with previously dechlorinated aged water. 

Water Change With Gravel Clean

Get your hands on some gravel vacuums because cleaning your gravel (substrate) while also conducting a water change is the best way to ensure that you have a clean aquarium! 

Uneaten food and fish poop gather up in the substrate and can raise the nitrates level. Therefore, it’s essential to remove these things from time to time.

The size of the gravel cleaner you choose should depend on how big your tank is. If it’s too small, then it will take forever to get rid of all that water. 

On the other hand, too large can make siphoning tough because they may not fit under some tank’s lids, have issues starting up, or they may also empty the tank very fast.

Read this article to know how to clean gravel with or without a vacuum.

Are There Any Other Ways To Change Water?

Bailing water out can be done with a jug for small tanks. On the contrary, you can use a bucket for large tanks, although it’s a hard physical job. 

If you don’t want to hurt your back, just pump water from the tank. Use an aquarium powerhead or a pump with a hose to remove the water from the tank. 

Wrapping Up

Fish tanks are like small aquatic ecosystems, and maintaining them can be a challenge for even the most seasoned aquarist.

Achieving healthy fish growth in your tank starts with ensuring it has all of its necessary nutrients–which means giving those poor dears some love by water-changing often! 

Freshwater is crucial for keeping your fish healthy and happy. You should change the water regularly to remove unwanted substances that could harm them, such as nitrates and phosphates, tannins, and staining from organic matter. 

Fish of specific breeds prefer certain pH levels or KH amounts, so if you’re not careful about matching up, that will hamper their reproductive system. 

Sujit Modak

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