Keeping your saltwater aquarium thriving isn’t just about feeding your fish and keeping the lights on—it’s about maintaining the right water quality. One of the most effective ways to do this? Regular water changes! If you’ve ever wondered how to do a water change properly or why it’s so important, you’ve come to the right place.
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.
