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Q10779 - HOWTO: How to Mix Saltwater and Perform a Water Change (updated 8/3/10) by Scott Brang, Reef Squad


Today’s aquarist takes every precaution to keep their aquarium healthy and looking great.

After all, this hobby is a large investment in time, money and effort. One of the greatest factors, often overlooked, is water.

Clean, pure saltwater is the basis for everything in your aquarium. Whether you are battling hair algae or have experienced a string of mortalities, the problem can often be traced back to the water you are putting into the tank. Not only is it important to look at the quality of water used, but also the quantity and frequency of water changes as well.

Our tap water supply contains many different chemicals to make the water safe for human consumption. These same chemicals can, however, be deadly to your tank.

Captive Purity 4-stage Pro RO/DI System
The worst of these toxins are chlorine and chloramines. These additives kill bacteria and other organisms in tap water, but even in low concentrations, will burn the gills of your fish.

It is therefore necessary to pre-treat water before it goes into our tanks.

For a fish-only system, a water conditioner will adequately neutralize chlorine.

For reef tanks, there are other chemicals in tap water that can harm invertebrates, like copper and heavy metals. For these systems, reverse osmosis (RO) filtration is strongly recommended.

Mixing Saltwater

Correctly mixing saltwater for a marine or reef aquarium is a vitally important—yet relatively easy—task.

First, find a container that can hold (at least) 5 to 10 percent of the volume of water used to fill your “display” tank. Old aquariums and clean garbage cans are popular among hobbyists, but you can choose whatever works best for your own “mixing tank.”

Next, fill the container with dechlorinated or reverse osmosis (RO) freshwater. It’s a good idea to aerate RO water for 12-24 hours before mixing in the salt. This will drive off excess CO2 which ensures a proper pH and buffering capacity in the synthetic seawater.

Marine Depot Refractometer
Add synthetic salt, being careful to follow the manufacturer’s instructions so you can match the salinity of your mixing tank water to your aquarium water. Ideally, salinity should be 27 to 35 parts per thousand (ppt), or 1.020 to 1.026 specific gravity. Use a refractometer to measure salinity and adjust your level accordingly.

Freshly prepared synthetic seawater is very caustic (capable of burning or corroding), so it will need some time to “cure” before it can be used in your aquarium.

Allow the newly mixed seawater to sit in your container for a minimum of 24 hours before use. During that time, place an airstone or powerhead in the water to circulate and aerate the mixture. Lastly, drop in a heater to match the water temperature to that of your display tank.

The Water Change

There continues to be much debate regarding how much water to change and how frequently to change it.

I have tried every scenario and have learned through experience that a small-to-moderate water change every 1-2 weeks works best.

I know several people who prefer to change 25-30% of the water once each month. By the end of the third week, however, this maintenance routine begins to show its shortcomings.

I prefer a water change of 5-10% of the total tank volume (including sump) every 1-2 weeks. This schedule doesn’t allow nitrates to build up or trace elements to deplete too far.

To begin the water change, turn off your lighting to reduce the stress level in the tank. You may have to turn off some pumps as well, so they don’t run dry.

Be sure to have the replacement water ready at least 24 hours in advance. The temperature and salinity of the replacement water should match what is currently in your display tank.

While siphoning out water, use a gravel vacuum to remove as much debris from the tank as possible. Don’t worry if some of your corals are exposed to air for a short time; in the wild, corals are often exposed to air during low tide.

Python Gravel Cleaner
Once the desired amount of water has been removed, begin adding in the new water. Alternatively, you may opt to have a powerhead replace the water as you siphon to limit the likelihood your corals come into contact with air.

The Importance of Keeping Saltwater On Hand

Saltwater is an invaluable asset for marine and reef aquariums and is often over looked.

A hobbyist can spend countless hours and thousands of dollars (depending on how big the tank is) just to lose it all because he wasn’t prepared. That is why I’m a firm believer in keeping extra saltwater made at all times. In my years as an aquarist, in both public and private settings, I’ve seen countless scenarios where a reserve saltwater tank saved the day. From a tank full of octopus ink that needs an emergency water change to a complete tank failure that left $10K in corals high and dry, a backup saltwater tank is well worth the small cost and effort.

All it really takes is a container that holds at least 10% of your aquarium’s volume, a powerhead and a heater. Personally, I have two 55-gallon trash cans in my garage, one for saltwater and the other connected my RO unit and a float switch. This system makes mixing saltwater and performing water changes a breeze. Plus, I have plenty of water on stand by in case there is an emergency in any of my tanks.

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Approved Comments...
Would have liked more info on storing emergency salt water,like correct methods and how long it would be good for. That said great info and please keep it coming. Over time Ive learned to ask one question to as many people (That are in reef keeping ) and I have found that when most answers are the same or real close that it is good to remember and use then the other info you get really depends on what you believe and works best for you. When you come up with a new way or method please share and more than not youll help someone with the same questions or problems you had. Approved: 9/21/2012
I have a 200ltr back up water. I found that if I keep it to long ie 3-4 weeks it starts to cycle and the nitrate goes up. I keep the water in a dark place and have the water moving all the time. I use RO/DI water for mixing. Do you know why this is happening? Thats why I marked it down a bit. Cheers Mark Approved: 2/25/2011
Good article, I use Glacier water for $1.75 for 5 gallons and a drinking water container. I think I will go get another container so I can have the premixed water sit for 24hrs or longer. I havent done this. Comments on the Glacier Water? I have a 28gal and pull out a half gallon and replace with a gallon each week to top off evaporated water. Maybe its not enough. Not sure. Approved: 8/19/2010
Its a good article but theres nothing new in it so I gave it a solid score but not a chart topper. Approved: 8/10/2010
Ive done a lot of research/reading about this hobby and I already do everything this article mentions mainly because its true and it works Approved: 8/9/2010
Great article. Even though I have been keeping a 75 gal. salt water aquarium for 3 years, I am always searching out information and articles to learn and improve. This is the kind of information I would have liked to have when I started. The salt water hobby is one that you never stop learning in. Thanks, and keep them coming. Approved: 8/7/2010
Informative and accurate from my experience - thanks for sharing! Approved: 8/7/2010
it has mentioned somethings that i did not know Approved: 8/7/2010
Perfect timing. I am looking at setting up a RO/DI and salt mixing station at this time. The only thing holding me back is what I could use to hold the water. I was not sure what I could use that would not contaminate the water that I just out through the RO/DI unit. Approved: 8/7/2010
I agree, anything anyone can learn is always helpful. Thanks Approved: 8/7/2010
anything I can learn i think is a great asset. Thank you Approved: 11/20/2008
I lost EVERYTHING in my 105 gallon tank!!! Could not figure what went wrong...I just have water in it now. Im reading everything I can to learn more and how to do better for the sea I LOVE SOOO MUCH and her creatures. Thank you for this information alot!!! Approved: 11/20/2008
When using Reverse Osmosis water you must always aerate the water for atleast 12 hours before adding any salt or buffers. This drives off carbonic acid / carbon dioxide. If you dont do this, buffers in your salt mix can be depleated. With increased oxygen youll have increased ph, and with decreased oxygen youll have a decreased ph. Imagine the ph in our blood stream. As we hold our breath, carbon dioxide builds up, and the ph in our blood stream drops. The same goes for the opposite. If we take in to much oxygen or hyper ventilate our ph will rise in our blood stream. In both cases we can pass out, if balance isnt restored. It is very important that stability is maintained in both our aquariums and our bodies. The point is, if you are using RO water, always circulate or aerate before adding salt or buffers. This is a brief summary. Hope this helps. Approved: 11/18/2008
It includes valuable information we all learn over time. Sometimes the hard way. Approved: 11/18/2008
A very nice article. I thought it was OK to use uncured saltwater, but this article was a wake up call. Approved: 11/18/2008
I am a beginner, so every little extra step I can take to minimize future mistakes is well worth the read. Thank you... Approved: 11/17/2008
Short and simple article but thorough and to the point. Funny I had never thought to keep a back-up batch of saltwater pre-mixed and ready to go at all times for 10% change and for emergencies - Ive always only mixed up a new batch when I planned (or needed) to do a water change. Thanks for publishing these helpful articles on your website and regular emailings. They are great for newcomers to the hobby, and are good quick-reads for "old timers" that need a reminder or two, or just a fresh write-up on an old topic that could use some new ideas. Please keep up the great work with Marine Depots "Education" page. Approved: 11/17/2008
Great information, I have one question? I have 144gl main display tank 1st floor and 75gl sump in basement/crawl space. Is there anyway that I can change water from the sump and not have to syphone from my main tank. My wife gets on me all the time and doesnt want anything but feeding from main tank. Also syphoning 30gls would be a big hassle for me becuase where the tank is located. Any suggestions would be great. Thanks (editors note...send us an email at for some reccomendations) Approved: 11/17/2008
great article, didnt know you should turn off the lights to reduce stress. Approved: 11/17/2008
I agree that water changes are one of the most vital, and yet underperformed, aspects of maintaining a healthy reef tank. I perform weekly water changes of about 10-15% of my total water volume - and started doing this because I had neglected to regularly do this important task for about 6 months. The result was a hair algae bloom out of nowhere that hit extremely hard and fast. After about 2 months now, the hair algae has subsided and the corals look great! I think the fish and inverts have also benefited from this routine. I have a mixing bin and top off reservoir connected to my RO system, and keep them full and ready all the time now. The time spent is well worth the results. Oh yeah -- maintain those RO filter cartridges and membranes, and refresh your DI resin regularly! Approved: 11/17/2008
I had no idea that you had to let saltwater cure. Thanks for the info! Approved: 11/16/2008
A good article especially for the beginners. Not sure stating "whatever works best for you" would be a wise statement in regards to a mixing container. Some may try to use a container that was used for something else that might have been toxic. May be better to state that the container should be new and/or never used for anything else. Personally, I like using an empty 5 gallon saltwater mix bucket. Also, the article should state that you add water first and then salt, and not the opposite. Keep up the good work. Kevin Approved: 11/16/2008
theres nothing wrong with big water changes. i perform a 70 gallon (or 35%) water change on my 180 (~200 gallon total volume) every two weeks. some well known "gurus" suggest even more. anthony calfo for instance recommends as large a water change as possible, as frequent as possible. the thought behind it being that corals and fish are exposed to 100% water changes every second with the rate and amount of water moving past them every second. Approved: 11/16/2008
you should never vacuum the sand bed in a saltwater aquarium. there are layers where anaerobic bacteria inhabit and where dangerous chemical compounds exist. if you disturb those lower levels, then you will get algae outbreaks and possibly even an ammonia/nitrite spike in your tank. Approved: 11/16/2008
Straight and to the point. A good article. Thanks. Approved: 11/16/2008
Didnt how long to cure water till now. Very Helpful. Thanks!!! Approved: 11/15/2008
Water quality is the one overiding issue I have encountered. Approved: 11/15/2008
I enjoy your e-mail newsletters ! I always learn something from them thanks for taking the time to do them. Don McGowan Approved: 11/15/2008
it has mentioned somethings that i did not know Approved: 11/15/2008
this article its great...personal i used purified water btls (i work for water btl deliverie) and i used pro coral red sea salt for mix and i change my water every wk on a 40g tank and my corals are great!! Approved: 11/15/2008
I never knew you had to have the temperture, from your "mixing tank", the same as your aquarium or turning of the lights, during the water change, to reduce stress! Thank you so very much for this informative article!! Approved: 11/15/2008
I did"nt know that you must let the saltwater mix for 24 hours before you put into your Tank. Approved: 11/15/2008
My experience is with reef tanks. I have had a 75 gal and nano tank for 8 years. I also have experienced the variety of water changes. I have found doing the 5-10% water change depending on the need of the tank the best method. For the larger tank 1-2 weeks is fine, the smaller tank I change the water 1-2 times per week. Most of the time my tanks are beautiful free from problems. Occasionally, like most aquarists I have had my run in with difficulty. Frequent water changes are always the mainstay of correction and preservation of the tank. It is all about import(of food) and export(of byproducts/waste). I highly recommend keeping the 1 full tank change of saltwater as it is like fine wine, better with age and you have this on hand for the next water change as well as any emergency that could crop up. Thanks to with Bob Fenner and his crew who have taught me most of how I manage my system. And, to Mr. Tullock and Mr. Fenner for their books which despite the internet Approved: 11/15/2008
This article is knowledge that is constantly repeated in all quarters of the industry. I would hope for more in depth knowledge of the parameters of salt water science. Approved: 11/15/2008
you could also talk about the calcium and alk associated with different salts and what is optimal to add to reef. Approved: 11/15/2008
Easy to understand. also because of time and amounts. I have reverse osmosios though and I still have a problem with hair algae cant seem to get rid of it. Approved: 11/15/2008
I think its great that you guys are doing this... Approved: 11/15/2008
I did not know salt water needed time to cure. Having back up water is an excellent idea also. Approved: 11/15/2008
I like the article because it gives times, amounts and proper preperation. Approved: 11/15/2008
I would suggest using a turkey baster or similar before performing the water change, to blow rocks of debris ,leftover food, and detritus. Cleaning the glass beforehand, might also be an idea.I would clarify RO/DI water should be used, not RO. A SG. of 1.020 is too low for corals and inverts, in my opinion.1.025-1.026 is best. I would also make a note that water should be topped off in between water changes, throughout the week, to keep SG. stable. Some people just dont know this. Carole Approved: 11/15/2008
It took me a long time to learn this simple truth...wish id know this from the start...simple but so true Approved: 11/14/2008
Article Details
Created on 11/11/2008.
Last Modified on 3/14/2013.
Last Modified by Dot Yuson.
Article has been viewed 61651 times.
Rated 6 out of 10 based on 419 votes.
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