How Often To Change Fish Tank Water in Your Aquarium

Charlie Morton

Charlie Morton


How Often To Change Fish Tank Water

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When I got my first aquarium twenty years ago, I just assumed that the filter would do most of the cleaning work and didn’t realize that I’d have to change the water frequently too!

But I soon learned that partial water changes are a standard part of good aquarium maintenance.

While aquarium filters perform an essential cleaning role in the tank, some additional cleaning and water changes help to keep the aquarium in good health.

In most aquariums, a 15-35% partial water change including gravel cleaning every one or two weeks is the optimum cleaning regime. But to know exactly what’s best for your tank, we’ll need to look at the topic in more detail. So let’s dive in!

Aquariums Need Regular Cleaning

Unlike a natural freshwater environment like a river or lake, a home aquarium is a contained body of water that doesn’t get replenished by water flow or rain.

With time, certain dissolved substances can build up in aquarium water and, if left unchanged, these substances can build up to dangerous levels.

To maintain healthy water conditions, regular partial water changes are needed to replace old, tired aquarium water with clean, fresh water from a tap or alternative water source.

Let’s take a closer look at the functions of partial water changes in more detail.

Why Do You Need Partial Water Changes?

Management of Nitrite and Ammonia Levels

Fish waste, leftover food in the tank, and decaying plant matter all produce ammonia in the water, which is then converted into nitrites by certain bacteria.

Even small amounts of ammonia or nitrites can cause ammonia poisoning or nitrite poisoning, so the target level of ammonia and nitrites in your tank is very simple: none.

You always want an ammonia and nitrite count to be at 0 ppm (parts per million) to keep your fish safe and healthy.

Thankfully, a good biological filter will do this job for you, with beneficial bacteria converting the ammonia and nitrites into less harmful nitrates.

But if your filter isn’t working properly and you have an ammonia or nitrite emergency, a large partial water change is one of the fastest ways to rectify the problem.

Moderate Nitrate Levels

Nitrates may be less harmful than ammonia and nitrites, but they can still be toxic in large doses.

Scientific studies have shown that nitrate levels as low as 44 ppm can affect freshwater shrimp and the most sensitive fish species – and levels above 80 ppm can affect most species of aquarium fish.

Very high nitrate levels can even be lethal to fish and also cause algae blooms which can lead to other problems in the aquarium.

Since nitrates cannot be removed by your filter, performing regular partial water changes is a good way to keep levels in check.

Moderate Phosphate Levels

Like ammonia and nitrites, phosphates are produced in an aquarium by fish waste, uneaten fish food, and dying plant matter.

And like nitrates, high levels of phosphates can cause algae blooms in the aquarium which can make the water green, acidic, and depleted in oxygen.

Phosphates don’t harm fish directly, but, to keep algae in check, phosphate levels should ideally be kept below 2 ppm.

Healthy live plants are a good way to reduce phosphate levels over the long term, but the most immediate way to reduce phosphates in the tank is by changing part of the water.

Keeps Water pH Stable

When you add new water from the tap, it begins to chemically interact with everything inside your aquarium.

Many of the biological processes within an aquarium make the water more acidic, and rocks, substrates, and driftwood can also alter the tank’s pH.

Over time, the pH of your tank could be shifted outside of the safe range for your particular fish or aquatic pets, so changing the water regularly is a good way to reset conditions closer to your desired parameters.

How Often To Make Partial Water Changes

How much and how often you’ll need to change your tank’s water depends on a few factors:

Size of Tank

Smaller tanks tend to need more regular water changes than larger ones because toxic build-up and changes in the water chemistry tend to happen faster.

Whereas 5 or 10-gallon nano tanks are renowned for having fairly volatile water chemistry, larger tanks tend to be more stable as there’s more water to buffer any changes that occur.

Number and Type of Fish

The amount of waste that a fish produces is known as its bioload.

Aquarium pets such as goldfish, Oscars, plecos, and turtles are renowned for being messy and producing a high bio load, whereas small community tank mates like neon tetra and zebra danios tend to produce a much lower bioload.

Large fish species with a higher bioload make their water dirty more quickly and so require more cleaning and water changes than small, low-bioload species.

But an even more important factor is your stocking density.

In general, a community fish tank should be stocked at a maximum of one inch of fish per gallon of water. If your stocking density is close to that limit, you might well need to change your water more often than if you had a sparsely populated tank.


How plants affect your water chemistry and how often you need to change the tank water is a bit more complicated. That’s because aquatic plants both absorb and produce waste products!

While live plants can reduce nitrate and phosphate levels in the water by absorbing them through their roots, dead plant material breaks down into nitrogenous waste and phosphates too.

If your plants are growing strongly and not producing too many dead leaves, then they will tend to have a purifying effect on the water, helping to keep it clean.

Plants that are struggling, on the other hand, will contribute waste products to the water, thus demanding more frequent cleaning and water changes.

Weekly or Bi-weekly Water Changes Are Optimal

How Often To Change Fish Tank Water

So now we’ve understood that how much and how often you’ll need to perform partial water changes depends on your tank’s size, inhabitants, and setup.

For nano tanks, or tanks with a high stocking density, partial water changes of between 15-25% should be performed weekly.

For larger tanks and those with a sparser stocking density, bi-weekly water changes of between 30-35% should suffice.

Some advanced aquarists with heavily planted aquascape tanks can sometimes manage to go longer periods without changing the water, instead utilizing the cleaning power of the plants and manually removing dead leaves to maintain safe conditions.

This method, however, requires regular water testing, abundant plant growth, and skillful management, so can’t be recommended for a typical aquarist.

Avoid Changing More Than 40% Of Your Water at a Time

Your aquarium’s water chemistry and bacterial ecosystem is a delicate thing, and your fish adapt themselves to get used to it.

If it changes suddenly, your fish could become stressed or even shocked and may suffer adverse health problems as a result.

That’s why, in normal circumstances, we wouldn’t recommend water changing more than 40% at any one time. If you continue to make water changes at regular 1-2 week intervals, harmful substances shouldn’t build up enough to necessitate larger changes anyway.

Emergency Water Changes

If your tank’s filter is broken or you suffer an ammonia spike, you may need to perform a larger, emergency water change to temporarily return the water quality to safe parameters.

In this case, you can break the 40% rule, remove 50% of the water and replace it with purified, fresh tap water.

When performing larger water changes like this, it’s important to get your replacement water’s temperature to perfectly match the water in the aquarium to avoid thermal shock in your fish.

How To Do a Partial Water Change in Your Aquarium

Gather Your Gear

You’ll need a bucket, a siphon tube, an old towel, a thermometer, and ideally a gravel cleaner (aquarium hoover) to attach to the end of your siphon tube too.

Begin Siphoning

If you have a primer pump on your aquarium vacuum then you can just press a button to get the flow started.

If not, you can use gravity to get the flow started or, as a last resort, suck on the tube, being careful not to get any of the water in your mouth (I speak from experience, yuck!)

Siphon off 20-35% Of the Tank’s Water into Your Bucket

If you have a gravel cleaner, dig it around in the gravel to disturb and suck up any detritus that’s stuck in there. Mop up inevitable spillages with an old towel.

Discard the Spent Water

Dirty aquarium water can be a great fertilizer for watering house and garden plants, and can also be used later to clean your fish tank filter)

Fill Your Empty Bucket With Clean Water

Mix cold and warm water together and check it with a thermometer to make sure it matches the temperature of your aquarium water.

Treat Your Tap Water With an Aquarium Water Conditioner That Contains a De-chlorinator

Chlorine can be lethal to fish, so don’t skip this step if your tap water contains chlorine!

Be patient and wait until the water purifier has done its job (some people like to do this before removing water in a separate bucket to allow more time for dechlorination).

Alternatively, you can use bottled drinking water.

Pour the Fresh Bucket of Water Carefully Into Your Fish Tank

Using your hand to deflect the flow and taking care not to disturb the fish, plants, and décor, gently pour your fresh water into the tank.

Repeat filling the tank with buckets until your tank is full. Your partial water change is complete!

Expert’s Tip: Test Your Tank’s Water Monthly

As part of your routine maintenance, it’s a good idea to test your tank’s water parameter’s once a month or so with a good water test kit.

API’s master test kit reliably tests your aquarium water for pH, ammonia, nitrites, and nitrates – some of the most important components of your tank’s water chemistry.

API Water Test Kit
  • Accurately monitors 5 most vital water parameters levels in freshwater aquariums: pH, high range pH, ammonia, nitrite, nitrate
  • Designed for use in freshwater aquariums only
  • Use for weekly monitoring and when water or fish problems appear

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Getting to know your aquarium’s water chemistry and how it changes when you clean your tank and change your water can help you establish the perfect cleaning and maintenance schedule.


The size and frequency of water changes in your aquarium depend on the size of your fish tank, your fish, and your tank setup – but broadly speaking a 15-35% partial water change should be carried out in most aquariums every 1-2 weeks.

While advanced fishkeepers have found ways to circumvent frequent water changes, for most of us, it’s a regular part of making sure our fish and aquatic pets remain healthy, happy, and at their beautiful best.

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