How to Mix Saltwater

It’s easy to assume everyone knows the basics after being in the hobby for so long, so I felt it would be best to go back-to-the-basics for a few posts.  I was shocked recently to read about so many people purchasing premixed saltwater as they didn’t know how economical and easy it is to “do it yourself.”  With typical usage, DIY saltwater is more economical than purchasing premixed saltwater for aquariums 40g or larger (see below for assumptions.)

Equipment / Supplies:

  • 1 mixing container slightly larger than the ideal water change size
  • 1 heater rated for mixing container size
  • 1 powerhead rated for mixing container size
  • 1 refractometer (preferred) or a hydrometer
  • 1 thermometer
  • Salt (enough for the water change)
  • RO/DI water (enough for the water change)


1.  For ease of numbers, let’s say your aquarium volume is 100 gallons, and you would like to do 10% weekly water changes.  This means you would make about 100 gallons of saltwater initially plus another 10 gallons weekly for the water change.  You would probably want your mixing container to hold 15 gallons to prevent water from splashing out, even though you would only fill it with 10 gallons of water.  I do not recommend mixing saltwater directly in a new tank as many salts produce a scum that is difficult to remove from a tank.  The photo below shows some of the impurities left in the bottom of my salt mixing bucket after my last batch of saltwater.  There’s no point in allowing that to go into the aquarium.

My 20g Brute Trashcan on Wheels

My 20g Brute Trashcan on Wheels

2.  The typical recommendation for heaters is that you’ll need 5 Watts for each gallon of water.  So, if you have a 10 gallon container of mixing water, you’ll need a 50 Watt heater.  I mix my water in an unheated basement, so I have to use a 100 Watt heater (so you may need a larger or smaller heater than recommended.)

Spare Heater I use for Mixing

Spare Heater I use for Mixing

3.  Most powerheads are more than adequate for mixing saltwater in all except the largest mixing containers.  I usually use whatever I have lying around.  Old MaxiJet and Koralias are excellent.  Use caution when using large pumps (like the Mag-series) as they are designed to move water linearly (like through a plumbing system) and may cause water to shoot out of your container.

Old MaxiJet I use for Mixing

Old MaxiJet I use for Mixing

4.  I purchase salt in 5 gallon buckets, which will typically make 160-200 gallons of saltwater.  This is the most economical for my situation.  Salt choice is user-dependent.  For fish-only tanks and low demand reef tanks, Instant Ocean is an inexpensive option.  I use SeaChem’s Reef Salt since it is designed for higher demand reef tanks, but there are many other good options available.

5 Gallon Bucket of Salt

5 Gallon Bucket of Salt

5.  I highly recommend using reverse osmosis deionized (RO/DI) water, but I have seen successful reef tanks that have only used RO water or even de-chlorinated tap water.  You’ll need enough for your water change.  For more information about RO/DI systems and water purity, see a basic tutorial here.

Economical RO/DI Unit

Economical RO/DI Unit

6.  Read the salt manufacturer label to determine how much salt you’ll need for the water change.  Each salt is different.  If you don’t feel like calculating it out, you can just add a little bit, then add more water or salt as needed after testing.  I personally run my aquariums at 1.026-1.027 specific gravity (SG) (around 35 ppt salinity.)  Fish-only aquariums (and even some low-demand reef tanks) can withstand a lower SG.  For a basic tutorial on salinity and specific gravity, see the post here.

7.  Add the recommended amount of salt to the mixing container.

8.  Carefully add the RO/DI water amount you need.

Adding RO/DI Water to the Salt

Adding RO/DI Water to the Salt

9.  Add the powerhead (adjust the flow level if able/necessary) then turn it on.  The water will probably be rather cloudy.  This is normal.  The water will clear over the next 24 hours.

Saltwater Mixing

Saltwater Mixing

10.  Add the heater and turn it on.  I always recommended unplugging the heater whenever you need to touch the saltwater to prevent injury.

11.  Allow the saltwater to mix for at least 24 hours.  This time allows the water to mix sufficiently with the air (and thus stabilize the pH.)  You may also notice the water has some brown scum.  Some salt mixes mix “dirtier” than others.  These are just impurities in the salt mix, and they typically settle or cling to the sides of the mixing container IF the salt mix was stirred long enough.  I’ve never had an issue with “dirtier” salts.  You may also notice some white hard flakes, especially on your heater and powerhead.  These are calcium deposits caused by a chemical imbalance in the salt mix.  The deposits are just the result of the chemicals balancing out.  These typically do not cause a problem other than the equipment needing more frequent cleaning.

12.  After the saltwater has mixed, test the salinity, and adjust it as needed (add more salt/water to achieve the necessary level.)

13.  Test the temperature.  I run my aquariums at 76 degrees Fahrenheit, although my main aquarium reaches almost 80 degrees due to the lighting (I’m working on stabilizing this.)  You’ll want the saltwater to be near your aquarium water temperature.  Adjust the temperature as needed.

14.  Perform other testing (optional).  Ideally you would test/adjust the pH, alkalinity, calcium, and magnesium before performing the water change.  I know very few aquarists who do this, but I do recommend it.  I do not do this step for my quarantine and grow-out tanks, but I do this step for my main display.

15.  Your saltwater should now be ready.

Economic Calculation:

In order to determine the break-even point for most aquarists, I used a few basic assumptions.  Premixed saltwater usually sells for $0.80 – $2.00 per gallon (USD).  I used $0.80.  A good RO/DI unit usually costs around $150 online, and tap water for the RO/DI is usually less than $0.01 per gallon (depending on location).  Instant Ocean salt can be found for about $45 for a 200 gallon-mixing box (of course, salt prices vary greatly depending on quality, amount purchased, etc.)  I assumed the tanks would be initially filled with the saltwater and then have the water replaced at the rate of 10% weekly.  I did not add in the costs for RO/DI water to replace evaporated water as this amount varies greatly with location and tank setup.  A high evaporation rate would make an RO/DI more economical.  I also assumed the aquarist would have supplies like extra heaters, powerheads, mixing containers, a thermometer, and a refractometer/hydrometer lying around.  I also did not factor in the cost of driving/delivery costs of saltwater or the time required in either process.

Break-even Points:

Using the assumptions above, I’ve given a few break-even points below to help you make an informed decision:

10g aquarium system:  approximately 4.5 years

20g aquarium system:  just over 2 years

30g aquarium system:  almost 1.5 years

40g aquarium system:  approximately 1 year

50g & larger aquarium systems:  less than a year


2 pings

  1. Fatman

    Great outline on how to do a water change. Can you explain how you would buffer and adjust the pH, calcium and alkalinity to the new mix before doing the water change?

    1. admin

      Thanks Fatman, I sure will! I’ll use that as the topic for the next post. It involves a bit of extra testing, but it really helps maintain stability, especially in SPS-dominant reefs. Thanks again!

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