You can create the perfect saltwater mixing container to have available for emergency water replacements as well as your regular water changes.
Make sure the 55-gallon trash container (available at any hardware store) is very sturdy and does not have any holes or metal parts in the container. Rinse it well with tap water. Do not use chemicals, cleaners, or detergents.
Place the container next to the aquarium and fill it with the amount of water you will need. Note: Do not fill it to the top since you may need room to add more water if you accidentally add too much salt.
Set the floating thermometer in the aquarium. Set the powerhead, along with the heater, in the bottom of container. Plug them both in. Take note of the tank temperature on the floating thermometer, then place the thermometer in the mixing container. Slowly add about 1/3 of the salt you will need. When the salt is dissolved, add another third. Check the thermometer; adjust heater if needed. When all the salt is dissolved, test the salt level with the hydrometer or refractometer and check the temperature. You will want these levels to be the same as the aquarium. Add salt if the specific gravity is too low, or water if specific gravity is too high, and adjust the heater if needed.
Once these levels match the aquarium, unplug the powerhead and heater. Attach one end of the tubing to the output of the powerhead and put the other end in the aquarium. Plug the powerhead back in (making sure the tubing will not jump out of the aquarium) and pump the new salt mix into the aquarium.
Moisture can turn salt mix into one large salt rock. Always make sure the container holding the salt is well sealed as soon as you are finished with it. Do not store the salt measuring cup in the salt container as it often collects moisture while being used. Bags of salt are best stored inside a plastic container with a good seal.
Using a refractometer
Refractometers are an incredibly accurate way to measure salt levels, even at very low levels. Not only can refractometers be used for saltwater aquariums, but also for African Cichlid and Goldfish aquariums that use much lower levels of salt. They compensate for temperature AND are very easy to use. Just lift the plastic lid from the angular end, place a drop of water on the surface, and close the lid. Look through the viewing lens (round end) as if you are looking through a telescope. The line where the blue and white areas meet is your salinity (concentration of salt in water) or specific gravity, depending on which scale you use.