Saltwater Aquarium Water Change (Percentage & Frequency)

Image Credit: colorblindPICASO via Flickr
Image Credit: colorblindPICASO via Flickr

Saltwater Aquarium Water Change (Percentage & Frequency)


A reef tank is a saltwater aquarium that attempts to mimic a natural marine environment’s conditions and chemical makeup. With reef tanks maintaining a delicate ecosystem, it is highly beneficial to the health of your tank that you perform routine water changes.

In ideal circumstances, these reef tanks promote the formation of symbiotic relationships between their inhabitants and allow them to thrive. However, unlike in the ocean, water does not flow freely as an automated method to keep chemical imbalances in check. As aquariums are closed ecosystems, aquarists need to manually reproduce this effect by replenishing elements and reducing certain nutrients within the tank.

Python No Spill Siphon
Instant Ocean Salt

How do you do a water change in a saltwater tank?

Doing water changes in a saltwater tank isn’t too different than doing one in a freshwater aquarium. The main differences are:

  • How to clean the sand substrate instead of a heavier gravel substrate
  • Making sure not to shock the more sensitive fish and corals with drastic temperature swings

Step 1: Gather everything you need

You’ll need to make sure you have the following before you attempt a water change:

Step 2: Heat your freshly mixed salt water (optional)

Consistent water parameters are best no matter what parameter it is. The same is true with temperature. If your new saltwater is unusually cold or if you’re changing a large volume of water it would be wise to heat the water to the same temperature as your aquarium. Dropping a powerhead and a heater into the container you’re keeping your new saltwater in should heat it up fairly quickly. Once it’s up to temp you can move on to the next step.

If you’re replacing a small volume of water, this step can be skipped. 20% water changes or less aren’t going to be enough to cause a major temperature swing that would shock your livestock.

Step 3: Drain the appropriate percentage of old water

For this step, you’ll need a siphon and a bucket, or you can use something like the Python No Spill Clean and Fill which I highly recommend.

The siphon is often started by sucking on the end of the siphon hose to get the water flowing down into the bucket. A tip to avoid a mouthful of saltwater is to scoop water with the vacuum end of the siphon to fill the siphon hose. This will allow you to get the siphon going without having to put your mouth on the hose.

If you’re using the Python siphon you can attach it to your nearest faucet, place the vacuum end in the aquarium, flip on the water at the faucet, and it will automatically begin sucking water out of your aquarium. The best part is it sucks the old aquarium water down the sink drain. No heavy buckets sloshing around and spilling as you try to dump them.

As you’re siphoning out the old water, plunge the vacuum into the sand and lift it back out. This will clean the sand. You don’t want to suck the sand out which is why you immediately lift the vacuum end back out. The sand is heavier than the organics that have settled in the sand and should slowly fall back out. You should see the organics continue to rise and get sucked out of the aquarium.

Step 4: Add the fresh saltwater

Once you’ve removed the target percentage of old saltwater you can now add the fresh saltwater back into the tank. Most people simply lift 5-gallon jugs and pour them into the aquarium. Depending on your situation, you may find it more reasonable to use something larger like a Brute trashcan. You won’t be able to lift this obviously so an inexpensive pump with a hose is a good way to get the water into the aquarium easily.

Reef tank water change percentage

Weekly or Bi-Weekly water changes are the most common and most suggested water change frequencies. More often is better but that can be too inconvenient for some so they’ll push out to larger percentages less frequently.

Below are the reef tank water change percentages listed by frequency:

  • Recommended Daily Water Change Percentage = 1%
  • Recommended Weekly Water Change Percentage = 10%
  • Recommended Bi-Weekly Water Change Percentage = 20%
  • Recommended Monthly Water Change Percentage = 50%

When should you do your first water change in a saltwater tank?

You should begin performing water changes once your aquarium has finished its initial nitrogen cycle. If you’re not familiar with the nitrogen cycle, it’s the process of your aquarium building up beneficial bacteria that break down fish waste and uneaten food into less harmful byproducts.

When ammonia and nitrite levels reach zero parts per million and nitrate levels are beginning to rise your aquarium has completed its cycle and it’s time to do your first water change.

The Importance of Regular Water Changes

When you create a reef tank, you have committed to caring for the living creatures that make it their habitat. The consequences of not administering regular water changes can range from the growth of unwanted vegetation to drastic increases in the mortality rates of your fish.

There are even times when your tank’s residents will present health issues that indicate a specific imbalance of your water. This includes the rapid growth of unintended plant life to physical blemishes to your coral or fish. It is always best to check your water quality and perform water changes before such problems occur.

Improves Fish And Coral Health

If you enjoy watching your fish actively swim around and take pride in their vibrant colors, then frequent water changes are a must. Fish, and most species of coral, are very sensitive to nitrite, nitrate, and ammonia. An excess of these compounds in your water will make your reef’s inhabitants sluggish, prone to disease, and harm the coloration of their scales.

Performing regular water changes are necessary to removing these harmful substances and balancing out the chemistry of your aquarium water.

Prevents Algae Growth

Algae is a problem that requires months to fight off completely and can ruin the aesthetic of your beautiful reef tank. It isn’t just algae either. There are many unwanted species such as dinoflagellates and cyanobacteria that can take advantage of abnormal nutrient levels in your tank and grow uncontrollably. Taking the extra time to regularly change the water in your aquarium will prevent this issue as an added side benefit.

Algae growth occurs when your saltwater contains abnormal levels of nitrate and phosphate. However, simply reverting your tank water to optimized levels will not be enough to get rid of this persistent, green invader. This is because the algae have already absorbed the necessary elements to grow and are storing them inside.

The best defense against algae propagation is regular water changes that keep your nitrate and phosphate levels down in the first place. However, if you already have this problem, then consistent manual removal of the algae will also remove the nitrate and phosphate they hold. The introduction of algae predators like Kole Tangs or Naso Tangs acts as an exceptional cleanup crew. Just be sure these new fish will not cause any problems for your existing tenants.

Elevates Aquarium Water Quality

Never forget that your reef tank is home to living organisms like yourself. This means that each healthy fish also creates a certain amount of pollutants in your water. These pollutants create discoloration in your water and, with enough accumulation, will cause your reef tank to emanate a foul stench.

This is not only a problem for the visuals of your aquarium, but can negatively impact the health of both fish and vegetation. Clean water allows for the direct passage of sunlight which is needed for coral, plants, and fish to flourish. A lack of sunlight will prevent photosynthesis and cause your aquarium to quickly degrade in the color variety you worked so hard to achieve. Even partial water changes have a massive effect on the transparency and health of your reef aquarium.

Replenishes Trace Elements

Trace elements are the components that comprise a fraction of a percent of natural saltwater. In nature, these trace elements are replenished through water movement and organic processes. Due to the incredibly small amounts of these elements in water, they are unmeasurable outside of the lab. Reef tank water changes mimic the effects of water transfer in nature making them one of the more effective ways of introducing trace elements into your aquarium.

Important Saltwater Parameters

Water changes are important primarily to keep saltwater parameters within a healthy range for fish and corals. You may need to do a water change earlier than you normally would if your saltwater parameters move out of the suggested range.

  • Salinity: 1.025
  • Conduct a water change if the specific gravity of your salt levels is outside of this range. This can be tested using a refractometer for higher accuracy or a hydrometer for a more budget-friendly option.
  • One benefit of regular water changes is that salinity will primarily be adjusted with proper salt mix ratios with freshwater.
  • Alkalinity: 7 – 12 dKH
  • Perform a water change if your alkalinity drops below 7 dKH. This measurement can be easily checked with a digital alkalinity checker.
  • Nitrates: < 20 ppm
  • Change out your water if your nitrates reach or exceed 20 ppm. This can be tested with a NO3 nitrate kit. Popular brands to look for include the Salifert or NYOS test kits.
  • Phosphates: < 0.1 ppm
  • Perform a water change if your phosphates reach or exceed 0.10 ppm. This can be tested with a PO4 Phosphate kit or a digital phosphate checker.

Hard coral requires stricter monitoring of your aquarium’s water chemistry. If your reef tank houses hard coral, then you should pay attention to the following measurement ranges as well.

  • Calcium: 400 – 450 ppm
  • Replace your water if your tank’s calcium levels drop below 380 or rise above 500 ppm.
  • Too little calcium content in your water will harm the growth of your coral while high levels can affect the alkalinity of your water.
  • Magnesium: 1250 – 1350 ppm
  • Change your water if your magnesium levels drop below 1200 ppm. This can be tested with a magnesium kit.
Tip: When mixing new saltwater for your aquarium, make sure you are using a salt mix that is specifically created for reef tanks. These salt mixes contain higher levels of alkalinity and other elements that your water needs.

You may be overwhelmed at first with the technical terms used when describing the optimal chemical composition of your tank. However, proper organization and scheduling can make this process second nature and will help immensely in recognizing the needs of your aquarium water. Here are a few easy tips on maintaining healthy living water for the aquatic inhabitants you’ve taken into your reef aquarium.

New Saltwater - Tools and Tips for Getting Started

There are two ways to get new saltwater for your water changes. The first is to obtain it premade from a local aquarium supply store. However, for aquarists with large tanks or who live in inconvenient places, transporting that amount of water every week isn’t feasible. The second method is to mix your own.

These tools are required to confirm that you are creating safe water for your reef aquarium.

  1. Small water pump
  2. Reef tank salt mix
  3. 5-gallon bucket
  4. Salinity Tester or Refractometer
  5. Small Heater
  6. Thermometer
  7. Fresh Water

Dechlorinate Your Water

If you are without access to purified RODI water, then tap water is also an option. However, you will need to take the extra step and dechlorinate the water since many cities add chlorine to their drinking water for safety concerns. It is also common for tap water to possess high levels of nitrates and phosphates. As was previously mentioned, this imbalance can lead to deteriorating health in your fish and vegetation. A dechlorinate that is strong enough to break down chloramines will prevent these issues.

Adjust the Salinity of the Added Water

Depending on the salinity levels of your tank water, you may want to add more or less of the recommended amount of salt mix to your freshwater. A general rule is to add half a cup of salt mix for each gallon of water you are replacing in your tank. This is correct when the salinity of your reef tank is already in the acceptable range, but if the salinity of your tank water is too low or high, the standard amount of salt mix will not correct it.

For example:

If your salinity is measuring at a low 1.02, then adding 20 percent of new saltwater with the standard salinity of 1.025 will not bring it back to the optimal range. Alternatively, if your aquarium’s salinity is too high from a poor initial filling, then you can dilute it with water that has a lower ratio of salt mix. Take into account the current salt content of your tank water and adjust your replacement water accordingly.

Clean Out your Sand

If your reef tank uses sand as a floor, then it could be trapping pollutants inside your tank. These appear as patches of discoloration in your sand and are a sign that it is time to do a thorough vacuuming to siphon out the waste. Allowing your sand to hold these pollutants will reduce the effect that water changes have as it can slowly release them back into your water.

Check the Temperature of New Water

Make sure to heat your newly mixed saltwater to a temperature matching your aquarium water. An exchange of 10-20 percent of your reef tank’s water is enough to cause a marked change in temperature that can shock and harm your fish. Although the change may not be enough to cause permanent damage, extra stress on your reef tank’s ecosystem is never welcome.

This is where the small heater and thermometer are needed. The ideal temperature for most reef tanks sits between 75 and 78 degrees Fahrenheit. Make sure not to let your new water rest before adding it to your tank as the temperature can fluctuate quickly.

How to reduce water changes in a reef tank

Water changes are one of the best ways to improve the quality of the water in your aquarium but they are a lot of work. Reducing the frequency that you need to perform regular water changes is very appealing.

If you can slow the decline of water quality you can reduce the frequency required to maintain good saltwater parameters. An oversized refugium with a good grow light is one way to help keep water parameters. Fast-growing macroalgae will consume much of the organics that harm water quality and lock them up in the macroalgae growth that can be later thrown away.

The other critical aspect a water change replenishes is trace elements. You can reduce the need for water changes by dosing trace elements. This requires testing to make sure you’re not over or under-dosing these elements.

These 2 main areas are the keystone of the Triton Method which eliminates water changes completely. Continue reading to learn more.

Another clever way of reducing the need for “manual” water changes is to automate the process to a certain degree. I cover that topic in the section on Easy Reef Aquarium Continuous Water Change System further below.

Are water changes necessary in a reef tank?

No, but they are by far the most common method of keeping water quality high and water parameters appropriate.

If you aren’t doing water changes to your aquarium you’ll need to ensure that you’re implementing other methods to properly control water parameters. For instance, an oversized refugium with fast-growing macroalgae can help consume unwanted nutrients that would otherwise lead to nuisance algae growth. Also, dosing trace elements such as magnesium that are normally added to aquarium salt mixes would be required to maintain these elements.

The Triton Method is probably the most well-known and effective way of maintaining a reef tank without water changes.

Is it possible to do no water changes in a reef tank? - The Triton Method

Water changes are the leafy greens of aquariums. They’re not something most of us enjoy doing but are a necessary evil in keeping the hobby (and our fish) alive.

The Triton Method was not designed with the intent to remove water changes. Instead, it allows for a more advanced process of filtration, lighting, and c that will minimize unwanted nutrients while giving you a better understanding of your tank’s chemistry.

Most of the time, frequent water changes are done just in case there is something wrong with the tank water. It is a preventative measure rather than one based on confirmed facts.

The Triton method uses an effective refugium, carbon to remove pollutants, and a series of reagents that control element levels. This is all capped off with an in-depth ICP test to fully explain the chemistry of your water and confirm that a water change is necessary.

In the end, it is theoretically possible that the Triton Method keeps you from ever having to eat the leafy greens of water changes ever again. However, the more likely result is that it will drastically reduce the frequency you do them by confirming when they are needed.

Reef tank daily water changes

Daily water changes are an excellent idea although most people don’t have the dedication to do them and therefore opt to do weekly or bi-weekly water changes.

1% daily water changes will help keep your saltwater parameters much more consistent and consistency is what fish and corals love. Larger water changes less frequently cause greater swings in water parameters which can stress fish. More frequent water changes with smaller amounts have the smallest impact on water parameters but done frequently they help keep them very consistent.

Weekly or bi-weekly water changes are fine but the gold standard is daily water changes if you can make the time to do them.

Easy reef aquarium continuous water change system

Many people have found that creating a continuous water change system is an ideal solution. By implementing this system you are automating the most dreaded maintenance chore for most people in the hobby while also making the biggest factor of water parameter swings almost flat.

To build a system to automatically change the water in your saltwater tank you’ll need a reef controller. There are several on the market but the Neptune Apex is considered the best choice by most, myself included. By adding the Neptune DOS to you can automatically pump out old saltwater and pump in fresh saltwater. The top-of-the-line Apex includes a salinity probe to allow you to ensure your salinity is maintained at 1.025 and you can schedule the DOS to regularly change small amounts of water with little effort on your part.

You’ll need a fresh saltwater reservoir and another to catch the old saltwater. You’ll occasionally need to swap these buckets or jugs when fresh saltwater is depleted or the old saltwater container is filled. It’s important to install float valves to monitor the water levels in these containers to ensure you are alerted when you need more fresh saltwater or need to empty out the old saltwater container.

Water changes during tank cycling?

Water changes are not suggested during tank cycling. Spikes in ammonia, nitrite, and nitrate happen because there are no beneficial bacteria to feed on those elements. An environment rich in one of those elements will encourage a bloom of beneficial bacteria that will feed on that specific element that is currently uncontrolled.

Once nitrate levels settle to appropriate levels you should begin a normal water change schedule.

When cycling a new saltwater aquarium you should be doing so without any fish or corals in the aquarium. The cycling process will create an environment that would be harsh for anything living, if not deadly.

Can you do too many water changes in a reef tank?

No, but you can change out too much water. It isn’t as important “how many” water changes you do but how much water you’re changing. Consistent and proper water parameters are most important.

Large swings in water temperature, salinity, nitrates, etc. can shock fish. You can do 1-gallon water changes every day and it will have a fairly small effect. But doing 1 90% water change would probably be too much if your water was poor. Most water parameters would be drastically different before and after the water change.

Water changes are meant to improve water quality. Frequent daily water changes are an excellent way to do that and the safest for your fish and coral. Rarely changing your water and doing large amounts is strongly discouraged unless you’re trying to solve a problem. 10% to 20% weekly or bi-weekly is more typical though.

Final thoughts

Some of these hitchhikers can be very useful members of your cleanup crew so don’t be too quick to get rid of them. Others are going to require dedication and determination to permanently eradicate.

No matter whether they’re harmful or beneficial, you need to be able to identify these rogue hitchhickers. Hopefully this informative guide has given you the data you need to help you identify what’s lurking within your rocks and crevices.

Best of luck to you as you identify and handle each of your live rock hitchhikers!

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