A Step-by-Step Guide to Mixing Saltwater

The foundation of any saltwater aquarium is, of course, the salt water itself, and through your time in this wonderful hobby, you’ll be mixing up plenty of it! You’ll need it when you first fill your aquarium and quarantine tank, whenever you perform a routine water change, any time you’re adding and acclimating new specimens, and for any number of unforeseen circumstances that might crop up along the way. It’s always wise to have plenty of extra salt water on hand.

Now, unless you happen to live on the coast and have access to a limitless source of clean natural sea water (which is actually more problematic than it might seem), you’ll need to make your own salt water using fresh water and a synthetic sea salt mix. Here’s how it’s done:

What you’ll need:

• Quality, reputable synthetic sea salt mix (we use Instant Ocean and Reef Crystals)
• Clean bucket or heavy-duty plastic storage bin (we use Rubbermaid Brute containers)
• Submersible aquarium heater rated for the desired volume of water (we use EHEIM Jager heaters)
• Floating or battery-powered thermometer
• Small submersible powerhead (or pump) for circulation
• Refractometer or hydrometer for measuring specific gravity/salinity
• Some type of stirring implement

Step 1

Fill your bucket or bin with clean fresh water. Remember, the salt mix will displace some of the water, so don’t fill the container completely to the top. Ideally, you’ll want to use water that has been purified through reverse-osmosis and/or deionization. Dechlorinated/dechloraminated tap water can be used for a fish-only system if you have excellent source water, but RO/DI water is still the safest bet.

Step 2

Take note of the manufacturer’s directions for a salt mix per gallon specification and begin stirring in sea salt slowly and methodically, testing the water with your hydrometer or refractometer to monitor the specific gravity (an indirect measurement of salinity).

For our purposes, we’ll assume natural sea water around the tropical coral reefs has a specific gravity of about 1.025 (salinity of 35ppt). This is a good target, especially if you have or plan to keep any corals or other invertebrates in your system. Fish-only systems can be maintained at a slightly lower specific gravity if desired, as low as 1.020 or anywhere in between.

Keep track of how much salt you’re adding throughout the process. That way, you’ll have a pretty good idea how much you’ll need to add the next time you’re mixing a batch of salt water.

Step 3

Once the desired specific gravity is reached, place the powerhead in the container and plug it in to provide circulation. Try to position the powerhead so its output creates some turbulence at the surface of the water in order to maximize gas exchange. Then, put in the submersible heater with the thermostat adjusted to the same temperature as your aquarium.

Step 4

Let the water sit, circulating and heating, at least overnight before using it. This will allow enough time for the salt to dissolve completely and the desired temperature to be reached.

Step 5

The next day, test the specific gravity again to ensure it’s still at the desired level. Adjust it up or down as necessary by adding salt or fresh water. Also, check the water temperature and adjust as needed.

If the levels are where you want them, you can go ahead and use the saltwater as needed to fill your aquarium, replace water following a water change, etc.

When initially filling a new aquarium, and before any livestock or live substrates (e.g., live rock or live sand) have been added, saltwater can be mixed in the tank itself. After that, it must always be mixed in a separate container.

IMPORTANT!

When water evaporates from a saltwater tank, all the dissolved solids in it (read: the salt) get left behind. Thus, top offs to compensate for evaporation must be made with purified fresh water, not salt water.