How To Mix Saltwater For Your Reef Aquarium

Through the evolution of keeping a reef tank, one thing that stays consistent is the need to perform water changes. Water changes remove waste and replenish the necessary elements your tank's inhabitants need to grow and thrive. Many of us start by purchasing saltwater from a local fish store for water changes, which does have its benefits—but also leaves some mystery in terms of the quality of water and what is actually going into your tank. Let alone the cost and effort of obtaining the water. 

If the store's filter system is not regularly maintained, there may be nitrates, phosphate, or other impurities in the water. Some salt mixes work better for fish-only aquariums while others with higher levels of calcium, alkalinity, and major/minor elements are better for reef aquariums. If a store changes salt mixes, it may also have unexpected effects on your aquarium.

Mixing your own saltwater at home is in fact a much better way to obtain saltwater because it allows you to choose the salt best suited for your aquarium and allows you to control many of the variables. You will also save some time and money in the long run. Thankfully mixing saltwater is fairly easy but must be done correctly and, of course, you will first need all of the necessary equipment.

Equipment List:

• Salt mixing container (minimum 10% of your tank's water volume)
• Refractometer or salinity meter
• Powerhead or mixing pump
• heater
• Thermometer
• Salt Mix
• RO/DI System

Your salt mixing container needs to be large enough to hold the amount of saltwater you need for a single water change, the bigger the better really. Saltwater stores easily and if you have the space for a large container, by all means. You can mix up a large batch every 1-3 months, then just use it as needed.

Plastic Brute trash cans - 20-50 gallons in size are super common. You can mix the saltwater directly in the container, get a lid to prevent debris from entering the container and the castor wheel accessory makes it easy to move the container through your house.  For small tanks, a simple 5-gallon bucket works. The container needs to be clean, food-grade plastic is the general recommendation. 

Having a second container to store freshwater comes in super handy too. Dedicated hobbyists will build a saltwater mixing station with two large water containers, RO/DI system and the necessary equipment they need for mixing and water changes. 

To measure the salinity we recommend the use of a refractometer or digital salinity meter for accuracy. A powerhead is needed to mix the water as well as an aquarium heater to help dissolve the salt and bring the water temperature up to match your aquarium. A separate thermometer is used to measure the water temperature.

When choosing a salt mix, you first want to find a brand that provides the parameters you are striving for in the tank. Or at least as close as you can get to your target parameters because this makes it easier to maintain those parameters with a regular water change schedule. Also, just pick a brand you trust and stick with it. A successful tank can be had using any of the major salt mix brands.  The worst thing you can do for your tank is constantly switching up the salt mix, this will lead to parameters swings and chemistry changes.  

How To Mix Saltwater

Step #1 - Fill It Up

First, fill the mixing container with purified 0 TDS fresh RO/DI water. You will want to mix enough water to complete the desired water change and I always mix a few gallons extra just in case the unexpected should occur. 

Step #2 - Heat and Flow

Next, drop in the powerhead to start aerating the RO/DI water.  This helps to remove excess CO2 and ensures a proper pH. You can also add the heater, let it temperature acclimate for 30 minutes, and then plug it in to start heating.

Step #3 - Salt Mix

Now look at the instructions for your salt mix and calculate the amount of salt needed to match the salinity in your aquarium. Each salt mix is different so be sure to read the manufacturer instructions carefully.

While the amount of salt is different between brands, the concept is the same. Just slowly add enough salt to reach the desired salinity level.

• 1.023 SG or 32 PPM - Fish only aquariums
• 1.026 or 35 PPM - Reef aquarium

For example: Brightwell Aquatics NeoMarine salt mix calls for ½ cup per gallon of fresh water for a salinity of 1.025 specific gravity. If we want to mix 20 gallons of water, that means we need 10 total cups of salt mix to reach 1.025 SG.

**Pro Tip: When storing salt mix, always be sure to reseal the container as best you can. This will avoid moisture making its way into the salt mix which can cause precipitation and turn your powdery mix into a solid block.  Always be sure your measuring cup is dry when scooping salt and avoid storing it in the container should it get wet. 

Step #4 - Add The Salt and Mix It

Add the salt mix slowly into the RO/DI water inside your mixing container. It is best to simply add one cup at a time, let it dissolve for a minute or two, then add the next cup. It is normal for the water to cloud when you add the salt mix but it will clear up. Once you have added all of the salt mix, let it mix for a few hours before measuring salinity. 

**Pro Tip: Never add salt mix quickly or all at once, this will cause precipitation. Always add it slowly, one cup at a time. Also, be sure to add salt into the water and never water into the salt.  In other words, fill the container 100% with water first, then add the appropriate amount of salt into the water. If you try to measure out the salt first, then fill it up with water, precipitation will occur.   

After 2-3 hours, if you are still not at the desired salinity level, just add a little more salt mix and give it another hour before testing again. Small adjustments can be made after the initial mixing pretty easily. Adding a small amount of freshwater can bring the salinity level down too.

**Pro Tip: If you overshoot your salinity level with too much salt mix by a significant amount, precipitation has likely occurred which means the elements are not balanced and it is best to start over. 

Once the desired salinity level is reached you should let the water continue to mix and aerate for the amount of time recommended by the manufacturer.  Some salt mixes are best used within 3-4 hours of mixing (Red Sea Coral Pro) while others should mix for 24-48 hours before use (95% of other salt mix brands). When mixing salt, always follow the manufacturer's instructions closely.  

Photo courtesy of Eddie Zia

Step #5 - Storing Saltwater

If you plan to store the saltwater for any period of time, you first want to be sure you have a tight-fitting lid. This will reduce evaporation and prevent contamination. Remove the pump and heater from the saltwater because you don't need to store the water with heat or flow. Tuck it away in an area that doesn't get too hot or cold.

When the time comes to use the water again, just plan ahead. Add the heater and powerhead back into your container at least 12-24 hours prior to your water change. This gives the water plenty of time to heat up and ensures proper gas exchange.