This photo is a royalty-free image from Pixabay.

“Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink.”

Samuel Taylor Coleridge, from The Rime of the Ancient Mariner.

For the benefit of beginners, I think it's time for a Water Change FAQ. I consulted with forum members to get their opinions, and I will include some of them where appropriate. This is not an exhaustive treatment of the subject. The comments in red are quotes from the forum members.

Just because there's an FAQ on a given topic doesn't mean you can't ask questions about it in the forum. You can always ask questions. Any questions. There are no stupid questions on our forum.

Here are some of the questions we get a lot on this particular topic in the forum:

What's a water change?

This is when you take out some of the water from your aquarium and replace it with new saltwater that you have already premixed. And, by the way, I'm not being facetious. People new to aquariums may not know much about changing water.

Why do we do water changes?

Water changes help to neutralize waste that you don't want in your aquarium by taking out some of the "used" water and replacing it with water that doesn't contain any of the waste chemicals like ammonia. It also enables you to remove some detritus that would otherwise dissolve into liquid waste. Water changes also allow you to replenish elements like calcium that are used up by your bioload. (1) (2)

Is this like "topping off"?

No. Topping off is replacing water that has evaporated. Note that water that has evaporated is plain water, the salt is left behind. So, when water evaporates, the water left in your tank becomes saltier, or the salt is more concentrated. That's why if you are replacing water that has evaporated, you always add just plain water, not saltwater.

A nine-month-old 30G for inspiration. The owner changes 13% of the water per week.

This photo is from the Reef2Reef archives courtesy of @pharazon. ©2018, All Rights Reserved.

How much water should I change? and how often?

Well, it depends on who you ask and also the age and maturity of your tank/system. There are also systems that require very few water changes or none at all. But the consensus seems to be at least 10% per week, especially for beginners. Smaller amounts more frequently is better than larger amounts less frequently.

Note that once you have livestock in your tank, you don't want to do a giant water change at one time unless you have a tank emergency like the anemones spawning in this tank. And the reason is because you are always aiming for tank stability with reefkeeping.

@PDR 1) How much do you change? - 2% Total water volume daily (14% per week.)

@lapin 1) How much do you change? 10 % per week.

@Crabs Mcjones 1) How much do you change? I change out 10% of the total system volume (around 6 gallons.)

@davocean How much do you change? I would say [for] a new tank it is very important to stay on top of WC's WEEKLY, at 10%, and this helps keep bad things from taking hold usually.

Can I mix the saltwater in my display tank?

Yes, if you're setting up your tank for the first time. Otherwise, no. Never.

Does it matter which brand of synthetic seawater I use?

Not really. They are all similar with slight differences. If you stick to a major brand, you're golden. Try a few and see what appeals to you.

The beach and open Pacific Ocean seen near Tofino, British Columbia, Canada, about 150 miles from where the author lives. Tofino has, according to Outside Magazine, the best surfing in North America.

This photo is a royalty-free image from Pixabay.

Can I use real seawater from the ocean?

Yes, you can with some caveats. You probably don't want to use water from close to shore because of pollutants. You don't want water near an estuary because of specific gravity (the saltwater may be diluted.) So, for starters you'll need a boat or long pier or jetty. And you'll have to get the water home, and it weighs about 8.5 pounds per gallon.

The one advantage is it may be cheaper if you need a lot of saltwater. I, personally, live five minutes from the ocean, and I wouldn't bother with it. I think it's more trouble than it's worth to me, anyway.

And you may need to filter it like with a several-micron filter or diatomaceous earth or something, and you may want to treat it with an UV sterilizer. It's a complicated subject beyond the scope of this simple FAQ. Ask on the forum if you want more information.

Should I heat the new saltwater before adding it to display tank?

It depends on how much you're changing and what the difference in temperature is. Most of the experts seem to heat it.

There is a formula for calculating what the new temperature will be, and you can see the change from the starting temperature of the display tank:

Think of it like a number line.

So, the lower temperature is T1_________X(new temperature)____T2 is the higher temperature.

(mass1) (Δt) (specific heat of liquid) = (mass2) (Δt) (specific heat of liquid)

(M1) (x - T1) (specific heat of liquid) = (M2) (T2 - x)(specific heat of liquid)

In this case, the specific heat of saltwater is the same between the display tank and new water going in, so you can cross out the (specific heat of liquid) on each side of the equation.

(M2) (T2 - x) = (M1) (x - T1) and solve for x, which is the new temperature.

Or I found a handy online calculator for this too. Or you can take my word for it. Or you can take the word for it from some forum members with a lot of experience:

@andrewkw 6) Do you heat the new water before adding it? Somewhat. I use an old pump that creates a lot of heat mixing but I don't actually check the temp.

@redfishbluefish 6) Do you heat the new water before adding it? Most definitely...if for anything, to get an accurate refractometer reading....don't trust the ATC [Automatic Temperature Compensation] on the refractometer. Cool water registers a higher refractive number.

@magikfly Do you heat the new water before adding it? Yes.

@Nick30G Do you heat the new water before adding it? No, I let the water at least come up to room temp. But i probably should heat it. I have never seen negative affects from it in my case. I would recommend beginners heat it to the tank temp.

@leepink23 Do you heat the new water before adding it? Yes and a power head as well.

A two-year-old 29G Biocube belonging to @John3 for inspiration.

This photo is from the Reef2Reef archives courtesy of @John3. ©2018, All Rights Reserved.

Do I add the water to the salt or the salt to the water?

Always the salt to the total volume of water being mixed.

Per @redfishbluefish:

Always add salt to the water, not the other way around. And, add the salt like you're salting popcorn. Don't "dump" a large volume of salt into water. Could result in micro environment of high alk/calc causing precipitation.

Use as much agitation as you can. (I use two old Koralia powerheads; a K4 and K8.)

Do not add more salt than the recommended amount the water needs. That is, as example, I make 25 gallons of water that needs 15 cups of salt. If I'm waiting for the last 5 gallons of water to be made, do not add that 15 cups to the 20 gallons, in anticipation of adding the last 5 gallons of water. Reason is the alk/calc concentration might be high enough to cause precipitation.

What do you recommend for beginners new to reefkeeping regarding changing water?

@EmdeReef Once a tank is cycled, it will go through a lot of ugly stages and water changes can help offset some. Weekly is a best practice IMO as it makes you stay in tune with your tank more.

@Orko Make sure the salinity [of new saltwater] is the same as your display tank.

@Gonebad395 Do your water changes. It really does help. Buy a brute container; it really helps with wheels if needed for bigger tanks and water changes. Also try different salts and see what works for you and your tank. There are plenty to choose from.

@NS Mike D Understand why you are doing water changes. Remember that a 20% water change will only remove 20% of what you are trying to remove (NO3 & PO4). New tanks consume much less nutrients than large mature tanks and we have the tendency to over feed our fish, so getting rid of this excess is important. As one adds corals and they grow that will consume more of the NO3 and PO4 that your fish produce as part of the nitrogen cycle thus out competing the algae.

Note from the Editor:

There are some very detailed articles about water changes coming down the pike. One is all about why we change water, and then there will be some about automatic water changes and how to set up for them.

(2) The need for water changes can vary significantly depending on your bioload and type of bioload. Some livestock, for example, eat the microscopic critters floating around in your water and don't do as well with pristine clear water. We'll address this in detail in future articles.


Special thanks:

Special thanks is due today to @eatbreakfast and @4FordFamily, who kindly read this FAQ and gave me comments before publication.


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Author Profile: Cynthia White

Cynthia received her BA in English from NYU. She has been a freelance writer and editor for over 20 years. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs, a five-minute walk from the Georgia Strait, the Pacific Ocean between Vancouver Island and mainland British Columbia, where the water temperature ranges from about eight degrees C (46F) in the winter to 15 degrees C (60F) in the summer. Bring your dry suit. And some hot coffee.