How Often Should You Change Aquarium Water? Main Factors

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Water changes are something all aquarists have to deal with. Old tank water holds nitrogenous waste and other pollutants. So it has to be removed and replaced with clean water. But just how often should you change aquarium water?

Water changes should be done once every two weeks for fish-only tanks that are not overstocked. There are many ways to do smaller or less frequent water changes as well. Including keeping fewer fish, feeding less often, and adding live plants.

Water change schedules are an intensive topic. Along with a few tips I’ve included this video to help further your education on the subject:

Table of Contents

How Often Should You Change Aquarium Water? Factors To Consider

Creating a schedule for aquarium water changes is not an exact art. They need to be done regularly. But the exact amount of water and timing depends on several factors.

As a rough rule of thumb, most aquarist with fish-only tanks should stick to doing a water change every two weeks. But this will depend on whether you have a heavily stocked tank, one that is under or over-filtered, whether holds exotic organisms like plants or corals, and so on!

Freshwater vs Saltwater Water Changes

The first thing to consider is what sort of aquarium water you have. Freshwater and saltwater tanks have different maintenance schedules. Planted tanks and coral reef tanks also do.

Generally speaking, saltwater tanks need a water change more often. This is because marine organisms are more sensitive to nitrogenous waste than freshwater ones. Coral and other invertebrates especially.

The ocean is one of the most stable environments on earth. Much more so than rivers, where rainstorms, floods, and minerals can shift water parameters in a short time.

Changes happen on a timescale of hundreds to thousands of years in the sea. Sudden increases in pollution harm saltwater animals faster.

Fish Population

How many fish live in the tank is by far the biggest influence on the water quality. As well as how big each fish is.

One good rule for the carrying capacity of a fish tank is the one inch per gallon rule. This one has been repeated for decades. And it really does have value so long as you are dealing with small fish.

The problem is when you try scaling it up. Fish body mass matters as much as fish length. Ten 1-inch neon tetras do not equal one 10-inch oscar cichlid in terms of their effect on a tank’s bio load. Ten 1-inch neon tetras would be comfortable in a 10 gallon tank. The oscar, not so much!

I always recommend under stocking an aquarium. Heavily stocked tanks need more water changes because more fish produce more ammonia. Which means a higher nitrite and nitrate level.

Fewer fish also means each individual fish has more room to swim. This is very important for territorial species like cichlids, bettas, gouramis, and some catfish.

It’s true that you can make up for overstocking a tank by changing the water more often. But fish will remain stressed and cramped, even if their water quality is good.

Aquarium Size

Aquarium size is not as big an influence on how often you should change the water in your fish tank. Roughly speaking, you would change the water in a larger aquarium less.

But that only applies to understocked tanks. Here is a table documenting some basic guidelines on frequent water changes versus aquarium volume:

Aquarium Volume	Water Change Frequency
1 to 5 gallons	Once Per Week
10 to 30 gallons	Once Per Two Weeks
40 to 75 gallons	Once Per Two Weeks
90 to 150 gallons	Every Two To Four Weeks
150+ gallons	Every Two To Four Weeks

Smaller tanks tend to have a larger bio load compared to bigger ones since there is more fish body mass relative to the water volume. So you need to change the water more often. But if you under stock the tank, fewer water changes are needed.

Types Of Filtration

The type of filtration and media you use is a major influence on the nitrogen cycle of your tank. The more effective your filter, the less often you need to do a water change.

You may have heard of “cycling” your fish tank when setting up a new aquarium. Cycling happens as beneficial bacteria colonize the tank. These bacteria live in your aquarium water but are found in the highest numbers in the substrate and filter.

Nitrifying bacteria consume ammonia and nitrite, converting these dangerous pollutants into nitrate. Nitrate then needs to be removed by you through tank water changes. Or by plants when they consume it as fertilizer.

The two most common types of aquarium filters are power and canister filters. Both use not just biological filtration. But also chemical media and mechanical filter media. The most common chemical media is activated carbon.

It doesn’t remove nitrogenous waste. But it does bind to tannins. As well as organic molecules that decay into ammonia.

Mechanical media is anything that physically screens particles from the water. Cotton floss is the most common. Specialty media like diatomaceous earth can also be used.

Undergravel and sponge filters are also useful for maintaining good water quality. But they are better used for specialty tanks. Rather than as your main filter. Some good uses for sponge filters include gentle filtration for quarantine tanks and fry rearing tanks.

The more advanced your filtration system, the fewer water changes you need to do. Keeping up with filter cleaning means changing the water even less often!

Can You Change Aquarium Water Too Often?

Most of the time I have to convince aquarists to do regular water changes. But is it possible to change the water too often? Yes, as it turns out. What happens if we change water more than we need to?

Water Chemistry And Too Many Water Changes

One issue with changing water too often is that your aquarium chemistry is impacted. Most fish have specific salinity, temperature, and pH requirements.

Without adjustment, your aquarium water parameters will start to resemble your tap water. Treated water in most countries is high in dissolved minerals and alkaline in pH (pH 7.0+). Some fish thrive in these conditions, including livebearers like guppies and African cichlids.

But most freshwater fish prefer acidic, neutral, or slightly alkaline parameters (pH 5.5-7.3). Live plants especially. Sudden shifts in chemistry can be dangerous, even if they stay in the safe zone of your fish.

pH, for example, is a logarithmic scale. Meaning when you go from pH 5.0 to 6.0, that is a tenfold increase in alkalinity. pH 5.0 to 7.0 is a hundredfold increase in alkalinity, which is fatal if done too quickly.

Low levels of nitrogenous waste are also helpful in some instances. A nitrate level of 5-10 parts per million is recommended for reef tanks, for example. Corals use it to feed their symbiotic algae (zooxanthellae).

If you do too many water changes in your reef or planted tank, the growth of many plants and coral species will halt. Especially when keeping corals that depend more on photosynthesis than actively eat.

Each time you are adding water, it should be at or near the same temperature as your aquarium. Using tap water that’s very hot or cold (+/- 5°F or more) compared to your aquarium temperature will stress or kill fish.

Do Water Changes Stress Fish?

Changing aquarium water will only cause stress to your fish if the incoming water has very different parameters. Too cold, too alkaline, too salty, and so on. Fish will adapt to most chemistry changes but this takes hours to days.

Frequent, large water changes are the worst for fish health. Frequent small water changes cause less stress and keep the environment stable over the long term. Plus they are less work for you.

Low levels of nitrate, tannins, organic molecules, and other “pollutants” are also beneficial. Animals don’t live in hyper-clean tanks free of algae, biofilm, and other signs of life.

So do perform water changes but don’t try to keep your fish tank spotless and crystal-clear!

Small, frequent changes of 10-15% are the least stressful to aquarium fish. Larger water changes (30-50%) should only be done if there is a major issue. Such as high ammonia levels or chlorine from untreated tap water.

Do Water Changes Remove Beneficial Bacteria?

Each water change will remove millions of bacteria from the system. But billions of them live in your tank. They are found in much larger amounts in your gravel and aquarium filter.

Filter maintenance and using a gravel vacuum will remove some. But not enough to impact the cycle of nitrogen.

So, generally, water changes will remove SOME beneficial bacteria, but not all.

One exception is doing a “spring cleaning.” What if you have a heavily stocked fish tank and clean the filter, vacuum the gravel, and do a big water change all at once? This can reset your cycle, causing new tank syndrome in your tank.

Ammonia levels can rise since you’ve lost too many of your bacteria. Once one fish dies, ammonia levels rise even more, setting off a deadly spiral.

Topping Off The Aquarium

Topping off a tank happens when the aquarium water level grows low. Evaporation happens constantly. Smaller tanks evaporate faster than larger ones since they have more surface area relative to their water volume.

Some aquarists take this as a sign that it’s time for a water change. When all you need to do is top off the tank with treated tap water. I don’t recommend allowing the water volume to grow very low, either.

As the water level drops, the concentration of salt, minerals, and pollutants grows. Only water molecules are lost to evaporation; everything else in the tank stays put. Putting added stress on the biology of your aquarium fish.

Last, as water evaporates, the minerals can form a white crust along the water line. Limescale is difficult to remove once its formed a few layers over the months and years. Top off the tank once water levels drop 10%. Or less, if you have the time!

What Happens If Aquarium Water Does Not Get Changed?

What happens if you were to not do a single water change? Ultimately, this would end in the death of your fish. In most situations, that is.

There is one kind of aquarium where you can do away with water changes forever! These are Walstad planted tanks. The method was designed by the ecologist Diana Walstad.

She wondered why fish tanks need so much technology to keep animals alive. And wondered if she could do away with changing aquarium water forever. After all, you won’t find a canister filter hooked up to a natural stream or pond.

Walstad aquariums use heavy plant growth and a soil substrate instead of gravel. Filters can be used but they aren’t required.

Nutrient/pollutant export happens through plant growth. Bacteria and plants consume ammonia, nitrite, and nitrate. And as you trim your plants you remove pollutants from the system for good! All you need to do is top off the tank with new water as evaporation happens.

But for non Walstad aquariums, we need to change water on a semi-regular basis. Otherwise fish health starts to suffer.

How To Reduce The Frequency Of Aquarium Water Changes

Doing a water change will remain a fact of life for fish keepers. But how do we reduce the frequency of this often dreaded chore?

Keep Fewer Aquarium Fish

The best way to reduce the frequency of aquarium water changes is to keep fewer fish. Fish are the source of all water pollution. Ammonia comes from their waste as well as uneaten food. Ammonia is also the source of nitrite and nitrate.

Once you know the carrying capacity of your tank, keep as few as you can. How few fish depends on your desires. But the less you have, the less often water changes are needed.

The less often filter maintenance will be done. And the less you’ll have to deal with algae and other problems.

Fish tanks with a few fish don’t have to look ugly or “empty,” either. Minimalist designs like nature and iwagumi-style aquascapes look fantastic with just a handful of fish! Well considered plants, rocks, driftwood, and substrate choices can more than make up for less fish.

Use Biological Media In A Filter

Aquarium ammonia, nitrite, and nitrate level depend on how effective your biological filtration system is. All tanks have beneficial bacteria in them, working hard to keep your water healthy for fish. But you can increase this number by adding more biological media!

Biological filter media is made from lightweight materials with huge interior surface area. Lava rock and porous ceramic bio media are the most common kinds. And if you have a reef tank, the live rock also acts as a place for bacteria to live.

Many power and canister filter models have bio media already included. But you can always add more. Especially in canister filters, which are designed with extra space for custom media additions!

We’ve talked in depth about nitrifying bacteria. But there are also a second kind called de-nitrifying bacteria. These consume nitrate, releasing it as nitrogen gas. most fish tanks don’t have many de-nitrifying bacteria because they are killed by oxygen.

Ceramic media with ultra-fine micropores like Fluval Bio Media can grow de-nitrifying bacteria, however. Allowing you to maintain a complete nitrogen cycle in your fish tank!

Add Aquarium Plants

Live plants are the second best way to reduce the number of water changes you need (after keeping fewer fish).

Plants use nitrogenous waste as food, locking it away in their leaves and stems. In fact, doing too many water changes makes growth difficult for plants.

Plants also consume carbon dioxide, releasing it as oxygen for fish to breathe. They also shade the water, lowering light levels so algae has a harder time growing.

Fish also feel more comfortable in lower light Brightening their colors in response and spending more time in open water!

To remove the waste that plants consume, simply clip back heavy growth every so often. This exports phosphates and nitrogenous waste from the tank!

Fast growing plants are the best for maintaining good water quality. These include hornwort (Ceratophyllum demersum), guppy grass (Najas guadalupensis), and common duckweed (Lemna minor).

Feeding Less Often

Good nutrient control is one of the most important skills that aquarium keepers have to learn. And feeding less often is a big part of this.

Feeding our fish is one of the best parts of keeping aquatic pets. But learning how much your fish need is important. Most fish only need to eat once or twice per day. And just enough food that they can finish in 3 minutes or so.

All fish will eat more if you offer it. But excess food goes to fat production and extra poop. Uneaten food falls to the bottom and gets trapped between gravel grains. Here, it rots, causing ammonia levels to rise until your next water change.

If you have a problem with too much uneaten food, you can add bottom feeders to help. Snails are a popular choice, though their numbers will explode if not controlled. Bottom dwelling fish like cory catfish, plecostomus, and kuhli loaches also eat leftover flakes.

Just make sure that they are still getting their fair share of fish food. All of these are active fish that will starve if not well fed.

Conclusion

You need to add water to your aquarium for many reasons. Including combatting evaporation and doing water changes with a gravel cleaner.

So it’s important to understand why you’re doing so. As well as the frequency and amount of water to remove per change. If you’ve made it this far, then you know just how much and how often to do a water change!

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Tank/Aquarium Guides