Mixing saltwater: it's easy and convenient. It's often even more economical than buying it, especially when you have a large aquarium.
I hate to say it, but it's been over two years since I bought premixed saltwater. It was a chore to drive to my local store and then come home with mixed salt, only to then begin the water change. The chore of going to get the water was my old excuse for not doing frequent enough water changes. (I have now developed more complex excuses... Did you hear about the time my RODI unit was sabotaged by a band of of rouge agents bent on world domination through the proliferation of cyanobacteria and destruction of coral reefs?!) Well the moral of the story (the purchasing of water not the rouge agent thing) was that its a mission to go get water, especially if buying large quantities of saltwater.
Besides the chore factor, the costs can add up quickly when you're paying someone else to mix water for you. While I can concede that it's less equipment and cleanup to just bring a couple of containers for fill up, I would not want to have to buy 30 gallons per week to change water on a 300 gallon system.
There is also a final consideration: emergencies. It's easier to always have a little saltwater on hand when you can mix it yourself. This is good in case something unexpected happens- and "it" will happen. Having water on hand might be the factor that saves your reef from failure.
So mixing your own water can be 1) more convenient, 2) less expensive, 3) good for emergencies. How hard is it? Not very. You'll need some supplied to get started. Here's your shopping list:
1) RO/DI water (owning a RO/DI system is ideal).
Your life will be easier if you own a RO/DI system to make your own water. This will give you easy access to freshwater for mixing saltwater AND for making top-off water. A good 4-stage RO/DI unit would be a good place to start, but anything above that is only better. Don't get too hung up on brands; they all work on the same principles. A TDS meter is a good add on (though not required) to help give you an idea of water quality and when it's time to change those filters.
2) A good quality reef salt mix.
This is probably the most important thing you will put into your system. A good reef salt is the basic recipe for all things good in your system: salt, calcium, alkalinity, magnesium, and minor trace elements. These are all important, and are often replenished via salt mixes. Using inferior salt (or saltwater) is a bad first step only preparing you for failure. A good quality salt mix made with RO/DI can really do miracles for your tank.
When selecting a salt, be sure to check for guaranteed levels. Most reef salt mixes will have a higher level of Calcium, Alkalinity, and Magnesium than your standard marine mixes. These can help keep you from needing to add more additives to your water, but realistically many "regular" salt mixes are fine.
3) Mixing container(s).
Food grade large Rubbermaid-type totes are good, many hobbyists use Rubbermaid trash cans. I use a pair of 5 gallon buckets or the rubbermaid-type of totes. Be careful to select something made of plastics that will not leach stuff back into your water. Most commonly feared is the leaching of phosphates back into the water. This becomes even more of a concern when putting pure RO/DI water into such a container as RO/DI water essentially becomes a sponge on a molecular level. (I'm not a scientist, don't quote me on that).
A good powerhead is your multitasking buddy when making your own salt water. It can mix the water and salt in your mixing container while also keeping the water aerated. A powerhead can also double as a utility pump to move water from your water containers to your aquarium (and vise versa). Use whatever fits, but try to get one with several feet of head pressure. I am able to use a MJ-1200 to pump water from my bathroom where I mix the water directly to my aquarium. Quite handy. You might also need some tubing (several feet) to actually connect to the pump when sending it to another room.
You will need something to check (and then double check) your specific gravity (often called salinity, but technically not the same measurement). I prefer a refractometer over a hydrometer, but if you already have a hydrometer, no harm. Remember that the refractometer or hydrometer are only as accurate as their calibration and care.
A heater placed in your mixing container will help get the temperature of the new water to the same temperature of your aquarium. The heater also gets the temperature up to appropriate testing temperature. Specific gravity measurements will be reflected differently based on the temperature of the water. This topic is much to large for the scope of this article, so I've included some references at the end of this article for further reading.
So we have our shopping list. Now lets get to work.
First, you'll need to make RO/DI water. I typically make enough for my saltwater changes and freshwater top offs at the same time. I started using 7 gallon water containers from 'Wally World' and a 5 gallon Sparklet's water jug. Only the best for Chromis' Reef. When I want to actually mix my saltwater, I'll add water to the 5 gallon bucket(s) or a large 16 gallon Rubbermaid tote I have.
Now is the time to add your powerhead and the heater to the water. Be sure to wait 15 minutes before plugging in your heater to prevent it from cracking. (Ever shocked yourself by cracking a heater and then sticking your hand in the water? Yeah... me neither. But it hurts. I've heard. Allegedly. Ok, it was me.)
Now we can add the salt. When using large containers of salt, be sure to mix the dry salt up to help ensure consistency. While it's economical to buy the 200 gallon salt mix, it can settle and thus give you weird results if you're not getting a good mixture. Ideally, buying smaller packages will give you better results. Now we can add the salt mix according to the directions. In GENERAL, you can add about 1/2 cup of salt mix per gallon of water. I recommend adding a little less than recommended as it's easier to add more salt to a bucket than to have to dilute it with water. The salt will begin to dissolve. I sometimes give it some help by stirring it or moving the powerhead around. You don't want salt to sit at the bottom. Let the salt completely dissolve. I would then allow the water to mix for several hours and then test for specific gravity. Add salt if needed. Ideal specific gravity for a reef tank is 1.023-1.026. I like to keep my levels around 1.025. This also ensures you get a good level of elements with the water as more salt will also bring in more calcium, alkalinity, and trace elements.
Test again when the temperature is ideal. Ideally, you will want to wait 24 hours to allow the salt to mix completely and give time for the temperature to stabilize. Test your new saltwater and test your aquarium. If they are a match, set forth in motion the water change procedure. If you're not ready to do your water change, leave the heater and powerhead going and the water will stay good for a good while. Be sure to test the water before use as evaporation may affect specific gravity.
So know that you've read this, hopefully you'll decide if making your own saltwater is for you. Mixing your own water can be convenient and economical. I prefer the convenience over cost as I technically always have access to saltwater when needed. Until next time...
 Refractometers and Salinity Measurement.
 How to Mix Saltwater and Perform a Water Change (with video).