I love water changes because they are simple, useful, and difficult to overcomplicate—unless we start pretending they fix every aquarium problem.
A water change can remove dissolved waste, dilute something unwanted, replenish elements, and give you a perfect opportunity to siphon out detritus. What it cannot do is diagnose the problem for you. If a fish is sick, a heater failed, the salinity is wrong, or you keep adding more nutrients than the aquarium can export, changing water may help without solving the cause.
This guide will show you how to do a saltwater aquarium water change from start to finish: choosing an amount, mixing replacement water, shutting down equipment safely, removing water without destroying the sandbed, restarting everything, and checking that the aquarium is actually okay when you are done.
What a Water Change Can—and Cannot—Do
How Much Water Should You Change?
For routine maintenance, a 10% to 20% change is a reasonable starting point for many beginner saltwater aquariums. I often prefer the larger end of that range because it removes more waste and gives me more room to siphon. But that is a starting point—not a law that every aquarium needs 20% every Saturday.
How much water you should change depends on why you are doing it, how much water is actually in the system, how closely the new water matches, and how stable the tank is before you begin.
| Why you are changing water | Reasonable starting approach | What should change the plan |
|---|---|---|
| Normal beginner maintenance | Start with roughly 10% to 20% weekly or every other week | Nutrient trends, stocking, feeding, other export methods, coral demand, and how well the replacement water matches |
| Visible detritus or heavy feeding | Change enough water to siphon the accessible waste without rushing | Sandbed age, livestock, low-flow areas, and whether debris will be stirred into the display |
| Rising nitrate or phosphate | Use the change as one part of the correction | The confirmed test result, feeding, stocking, trapped waste, filtration, and the size of change required to make a meaningful difference |
| Small aquarium with little margin for error | Use a carefully measured change with closely matched water | Total system volume, evaporation, salinity drift, temperature, and the animals present |
| Contamination, medication, ammonia, or another urgent problem | Verify the problem and follow a situation-specific response | Livestock distress, oxygenation, pH, salinity, temperature, treatment instructions, and expert or veterinary guidance |
| Long-neglected aquarium with unusual pH or ammonia | Stop and test before making one giant correction | Old-tank chemistry can make abrupt correction dangerous; use measured changes and experienced guidance |
When you can, do the math with the amount of water actually in the system—not just the number printed on the aquarium box. Rock, sand, rear chambers, sumps, and the normal operating water line all change the amount of water in the system.
If you need help estimating system volume, use the aquarium water-volume calculator. Then write the normal change volume on the storage container or maintenance log so you do not have to reinvent it every week.
Prepare the Replacement Saltwater Before You Remove Anything
The safest water change starts before the aquarium notices anything. I want the new water mixed, measured, and ready before the old water comes out.
- Use clean purified water and aquarium-only containers, pumps, heaters, tubing, and measuring tools.
- Follow the current mixing instructions on the exact salt you own. Mixing time, temperature, aeration, and storage are not identical across every salt.
- Add the salt to the water in the separate mixing container. Never add dry salt directly to a stocked aquarium.
- Let the salt dissolve and stabilize for the time required by its manufacturer.
- Bring the water close to the aquarium’s temperature when the size of the change and livestock make that important.
- Measure salinity with a clean, properly calibrated marine instrument. Correct the mixing container—not the display aquarium.
I also like to inspect the mixed water. If it has an unusual odor, residue, temperature, salinity, or appearance, I would rather stop and figure that out before it reaches the tank.
Use the beginner salt-mix guide if you are still choosing a mix. For the actual measurement process, the water-testing and salinity guide explains how to verify a result before reacting to it.
How to Do a Saltwater Aquarium Water Change
Decide why you are changing the water
Choose the amount before you start. Are you completing normal maintenance, siphoning a dirty area, reducing a confirmed nutrient trend, removing medication, or responding to a specific problem? That answer determines the amount, testing, and preparation.
If you plan to test the aquarium, take the samples before the water change. Otherwise, you are measuring the fresh saltwater you just added instead of learning what happened in the tank.
Check the new saltwater one last time
Check the volume, temperature, and salinity. Confirm the salt is fully dissolved according to its instructions. Put the clean-water hose or transfer pump where it cannot fall onto the floor or blast the sandbed.
Keep the clean-water equipment separate from the dirty-water equipment. I do not put the siphon that just cleaned the sandbed into the new saltwater container.
Get the aquarium ready and turn off only what needs to stop
Turn off or pause any heater, return pump, ATO, skimmer, dosing pump, or other equipment that could run dry, dose into a stopped system, overflow after the water level changes, or interfere with the job. The exact shutdown depends on the aquarium.
Keep appropriate circulation and oxygenation when the change will take more than a few minutes. Protect outlets and controllers from splashes, and never handle plugs with wet hands.
Move loose debris where the siphon can catch it
Clean the viewing panel first if it needs it. Use a turkey baster or small pump to move detritus off accessible rockwork and out of obvious low-flow areas. Give the mechanical filtration a few minutes to catch suspended material when that fits your routine.
Do not turn the display into a sandstorm. The goal is to remove waste, not bury coral and fish under everything you loosened.
Remove the amount of aquarium water you planned
Use a siphon or utility pump and a container with a known volume. Watch heaters, pumps, livestock, and the water line throughout the removal.
When possible, I like removing part of the water from a sump or rear chamber because it reduces disturbance in the display. If you are siphoning debris or cleaning sand, remove water from the display where the waste actually is.
Stop at the planned amount. More is not automatically better once the replacement water, equipment exposure, and livestock stress begin changing the risk.
Clean only the sand or bare-bottom areas that need it
On a bare-bottom aquarium, remove the visible piles and low-flow deposits. On a newer shallow sandbed, clean a manageable section without sending fine sand into pumps or covering animals.
Be much more cautious with an old, deep, or neglected sandbed. Do not aggressively churn the entire bed because a calendar told you to clean it. If the bed has dark layers, unusual odor, trapped gas, or years of accumulated debris, make a specific plan before disturbing it.
Add the replacement saltwater slowly
Add the new water where the flow can disperse it without blasting fish, coral, rockwork, or sand. A sump or rear chamber works well when the system design allows it. Otherwise, direct the stream onto a clean plate, rock, filter sock, or another safe surface.
Watch the display and the operating chambers as the level rises. Stop if the salinity, temperature, odor, appearance, or livestock response does not make sense.
Restart everything and make sure the tank is normal
Restore the system in the order required by your setup. Watch long enough for the water levels to stabilize. Confirm normal flow, temperature, salinity, ATO behavior, skimmer behavior, and dosing state.
Record the date, amount changed, salinity, anything you cleaned, and anything unusual. If the change was meant to correct a test result, decide when you will retest. Do not assume the problem is solved because the bucket is empty.
The restart checklist
- Return pump is running and producing normal flow.
- Heaters are submerged correctly and powered.
- Circulation pumps are running and not blowing sand onto animals.
- ATO is enabled only after the normal operating water level is restored.
- Skimmer is restarted, seated correctly, and not overflowing.
- Dosing pumps and controllers are back in their intended mode.
- No hose, valve, bucket, towel, or tool was left where it can cause a leak or contamination.
- Temperature and salinity make sense after the system circulates.
- Fish and invertebrates are breathing, swimming, and opening normally.
What Changes for the Three Beginner Tanks
DeskMate 4.8
A small change is a large percentage of this aquarium. Measure the amount, match salinity and temperature closely, and add the water slowly. The rear chamber and tiny return section can change level quickly.
Use the change to remove visible waste, but do not make the tank unstable by turning a simple job into a complete teardown.
HelloReef 15
Follow the course routine included with the kit. Prepare the saltwater before shutdown, protect the heater and return pump, service the mechanical filtration at the right point in the sequence, and verify salinity before and after.
The public guide is an overview. The full 50-plus-video HelloReef course provides the detailed setup and maintenance process.
JBJ RF-45
The larger volume gives you more room, but a 20% change also means moving more water. Plan the containers, hoses, electrical shutdown, and refill path before you begin.
Watch both return sections, two pumps, filter socks, heater placement, and any optional skimmer or ATO equipment during restart.
Common Water-Change Mistakes
The Water-Change Tools I Actually Use
You do not need a complicated water-change station to do this correctly. For a small aquarium, clean buckets, a basic siphon, a towel, and a trustworthy salinity tool can be enough. Larger systems make pumps and hoses much more valuable.
Water-Change FAQs
How often should I change water in a saltwater aquarium?
Weekly or every other week is a sensible place for a beginner to start. Adjust it using the aquarium’s actual nutrient trends, stocking, feeding, other export methods, coral demand, and the condition of the system—not because every saltwater aquarium needs the same calendar.
Is a 10% water change enough?
It can be for routine maintenance on a stable aquarium. A 10% change removes roughly 10% of an evenly distributed substance in that one change. Whether that is enough depends on the reason for the change and what is adding the substance back.
Can I do a 50% water change?
Large changes can be appropriate when the reason is clear and the replacement water matches closely, but they also magnify salinity, temperature, alkalinity, contamination, and equipment risks. Verify the problem and make a situation-specific plan before treating 50% as routine.
Should I test before or after the water change?
Test before the change when you want to understand what happened in the aquarium. Test the replacement water before it goes in. Retest afterward when the change was intended to correct a problem or when the final salinity, temperature, or livestock response needs confirmation.
Can I add salt directly to the aquarium?
Never add dry salt directly to a stocked aquarium. Mix it completely in a separate clean container using the salt manufacturer’s instructions, then verify the new water before adding it.
Should I vacuum the sand every time?
Remove waste where it has accumulated. A newer shallow bed may tolerate cleaning a small section during routine changes. An old, deep, or neglected bed deserves much more caution. Do not churn the entire sandbed just because water-change day arrived.
Why did my skimmer overflow after the water change?
Water level, additives, new saltwater, cleaning residue, and changes in surface tension can all affect skimmer foam. Turn the skimmer off before it returns waste to the system, verify sump depth and cup position, and follow the skimmer manufacturer’s restart guidance.
What if the fish look stressed after the change?
Check oxygenation, temperature, salinity, flow, contamination risk, and every piece of equipment that was stopped. Verify surprising measurements. If fish are gasping, collapsing, or rapidly worsening, treat it as an emergency and get experienced or veterinary help for the exact situation.
When you are done, return to the Weekly & Biweekly Maintenance guide or explore the complete Saltwater Aquarium Maintenance hub.
Source review: July 2026.
The procedure is grounded in the current instructions for the exact salt and equipment you own. Supporting references include the Red Sea Coral Pro Salt manual, the Merck Veterinary Manual maintenance table, and the Merck aquarium-fish management guidance.
Why Are Water Changes So Important?
If you’ve noticed issues like unhappy corals, nuisance algae, cloudy water, or even sick fish, there’s one simple maintenance task that can help—a water change. But why does this work so well?
Think of it this way: in nature, ocean currents constantly refresh water, maintaining a stable, clean environment for marine life. In an aquarium, waste and excess nutrients can build up over time, leading to problems. Performing water changes helps remove unwanted things from the water replenish essential things, creating a stable and healthier ecosystem.
The Science Behind Water Changes
Water changes are like a reset button for your tank. They remove excess nutrients like nitrates and phosphates, replenish essential minerals, and help prevent issues like algae outbreaks and old tank syndrome. But did you know they also set an upper limit on how much of a certain compounds can build up in your tank?
For example, let’s say you have a 100-gallon aquarium and perform a 10% water change every week. Even if you add 10 parts per million (ppm) of nitrate weekly, after 50 weeks your tank will have a maximum of 90 ppm nitrate. Without water changes, that number could climb to 500 ppm—a dangerously high level for your fish and corals.
If you increase your water change to 20% weekly, nitrate levels cap at 40 ppm instead of 90 ppm.
Additionally, water changes help maintain trace elements necessary for coral health, such as iodine, potassium, and strontium. These elements get used up over time and are not always replenished by food or supplements, making regular water changes even more beneficial.
If you want to fully understand the effects of a weekly water change, check out the reef calculators at Hamza’s Reef. I use the Water Change Calculators all the time!
How to Do a Water Change Correctly
Performing a water change correctly means causing as little stress to your animals as possible. Primarily, this means making sure your fresh seawater is mixed thoroughly, matches the salinity of the tank water, and matches the temperature of the aquarium.
Finally, you need to remove tank water and add back in freshly made seawater in such a way as to minimally disrupt your pet’s habitat. This means avoiding hugely disturbing the sandbed and maybe performing the actual water change in a sump or rear filtration chamber to minimize stress.
I will write out the instructions below, but feel free to watch this video I made for HelloReef. It should automatically start at minute 2:37 for you, which is where I get into the nitty gritty.
1. Choosing the Right Salt Mix
Choosing the right salt mix means knowing two things: important water parameters like calcium, alkalinity, and magnesium, and knowing your seawater mixing and storing habits. Some salts can be mixed and must be used within a few hours, while other salts need to mix for 24 hours, and still, other salts can be stored for weeks at a time.
I’ve already made a blog about my top five beginner salts, so click HERE to check it out.
Directly below are two of my favorites for beginners. The most budget friendly option is Instant Ocean Reef Crystals, but I personally use Tropic Marin Pro-Reef Salt because it mixes clean and I can store it for months without issue… it does cost significantly more though. You can click on each of the pictures below if you want to check them out over at Bulk Reef Supply.
You’re going to want to start by choosing a salt that matches your mixing and storing habits, and you just have to read the label on the seawater mix to determine this.
Knowing your calcium, alkalinity, and magnesium levels and how much they deplete each week is the second step to choosing the right salt mix. Some salts mix to 7.5 dKH, while others mix to 12 dKH. If you’re shooting for a range of 7 to 9 dKH, then choose a salt that will adequately replenish those numbers during your weekly water change.
Or, if you have enough coral and other inverts in your tank to necessitate you starting a two-part dosing regimen, then consider choosing a salt with lower calcium, alkalinity, and magnesium levels.
Unfortunately there is no right answer here… my best recommendation is to choose a salt and stick with it.
2. Mix & Heat Saltwater
It’s important to mix your salt water really well and also heat it up to closely match the aquarium tank water.
Every saltwater mix has different mixing instructions so, I can’t give you a one and done explanation of how to mix it up… unfortunately. Here’s an example from RedSea’s Coral Pro mix.
Why are the instructions so different depending on the salt? Because different seawater mixes have different levels of additives and some of them precipitate out quickly while others can maintain a stable level. All I can recommend is read the directions on the back of the saltwater mix and follow them closely.
One of the reasons I love using Tropic Marin Pro Reef Salt so much is because I can just mix it up, use it almost immediately, and store it indefinitely without the water chemistry changing. But if you don’t plan on storing your water, then don’t go with Tropic Marin because it costs a lot more!
I heat my water to 77° and keep a large 50 gallon drum of it stored in my house at all times. I do that so in case there’s ever an emergency, I can immediately set up another tank with saltwater that’s already warm. It’s also really convenient for weekly water changes because it saves me the hassle of having to mix up a fresh batch.
You can either mix up your fresh seawater by hand, or even easier is to pick up some sort of utility pump to do the mixing for you. I think the Sicce Syncra Silents are perfect for this. Put the pump in a bucket, plug it in, and will do all the mixing for you.
As far as the heater goes, I usually have an extra Eheim Jager sitting around, so I pop that in the bucket and heat up the water. Although to be honest, I’m a big fan of the slightly more expensive Sicce SCUBA Preset, because it is actually accurate and heats the water to 77 degrees without any hassle. It will actually be my new go-to going forward… it’s just that I still have a lot of Eheim Jager’s sitting around so I use them.
3. Use Gravel Vacuum to Remove 10%-20% Aquarium Water
When possible, I like to do larger water changes, closer to 20%. But doing a weekly 10% water change is still fantastic.
Before you remove any aquarium water, be sure you unplug all of the gear. You definitely don’t want your return pump or heater to run dry during this time!
Your goal during this process is to not stress out your fish or coral. Ideally, remove most of the water from either the sump or rear filtration chamber, as the fish won’t notice this at all. But since you will be doing a larger water change, you will likely have to remove some water from the display tank.
In that case, it’s a good idea to clean a small portion of the sandbed. I clean 1/4 of the sandbed each week, and do my darndest to not create a sandstorm.
If you have a smaller tank, a simple 5-gallon bucket will do the trick, but if your tank is larger, you may need a couple buckets or you could invest in one of these Brute trash cans with the dolly.
I’ve also found that while not at all necessary, this Sicce Ultra Zero Utility Pump is fantastic at transferring the water from the bucket or trashcan into the sink. It’s really cool because you just need to attach a garden hose to it and it can empty water down to a quarter of an inch. As I get older, it’s nice to not have to lift heavy 5-gallon buckets all the time! 🙂
4. Add Mixed & Warmed Seawater to Display Tank
Now that you’ve cleaned a part of the sandbed and removed 10 to 20% of the water from your display tank, it’s now time to add in the freshly made seawater. If you haven’t already done so, use a refractometer to test the salinity of your water. I always keep mine between 1.025 and 1.026 parts per thousand and that’s worked well for me.
There are a lot of different tools you can use to test the salinity of the water, and I have used them all. I’m just going to recommend this refractometer that comes with calibration fluid because you only have to calibrate it once and it is extremely accurate. The only thing to remember with refractometer is that they are temperature sensitive, so be sure you’re checking the salinity of the water in a warm room with water that is as close to 77°F as possible.
There’s no real secret to adding freshly made seawater back into your tank other than doing it as carefully as possible, so as not to stir up your sandbed. I like to do this by adding water into the rear filtration chamber or sump in my tanks. You can also do something like pour water over your rockwork or into a filter sock so it disperses more evenly throughout the tank.
The most important things here are to make sure the water is at the correct salinity, that you follow the mixing instructions on the saltwater mix container, and that the water temperature closely matches the aquarium water temperature.
Problems Water Changes Help Solve
While water changes won’t fix every problem in your tank, they can help prevent and mitigate several common issues:
High phosphates and nitrates (which fuel algae growth)
Low calcium, alkalinity, and magnesium (needed for coral growth)
Trace element depletion (essential for coral health)
Ammonia spikes (which can be deadly to fish)
Old tank syndrome (which happens in tanks that haven’t had consistent maintenance)
Accumulation of organic waste that can lead to poor water quality
The Misconception About Water Changes
I’m not saying a water change is going to solve every problem in your tank, but I do think consistent weekly water changes will get you 80% of the way there.
There’s an unfortunate misconception in this hobby that water changes are really only there to alleviate problems once they’ve already happened. But for me, that’s like saying diet and exercise are only needed once you have your first minor heart attack. That just doesn’t make sense to me.
Think of your weekly water change like diet and exercise—you’ll keep your tank healthy and prevent a whole bunch of problems from ever even happening.
Final Thoughts
Now that you know how to do a water change properly, you can see why it’s such an essential part of saltwater aquarium maintenance. By performing regular, well-executed water changes, you’ll keep your tank’s water quality stable, your corals happy, and your fish thriving.
So don’t wait until things go wrong—make water changes a part of your routine maintenance, and your aquarium will reward you with vibrant, healthy life.
