Water is a crucial component of the tank. It's very important to start out with the cleanest water possible: free of pollutants and harmful chemicals that can be deadly for our creatures.
Water from the tap or a well contains a number of minerals and chemicals that are measured as "Total Dissolved Solids" or TDS for short. Our goal is to have water that has zero TDS. Otherwise, we may run into algae or worse problems. Regular tap water is not suitable so we have to look for other alternatives.
If your tank is not very large, this can be the easiest option. Many fish stores sell saltwater that has been filtered appropriately and is ready to be used. They also sell containers to carry the water from the store to the tank. Larger pet stores sell saltwater in a box, which is pricier but very convenient. You can buy the initial amount of water you need to fill the tank and explore other options while you wait for the tank to be cycled.
Buying Filtered Freshwater
You can also choose to buy filtered freshwater and add salt yourself. You just have to make sure it has zero TDS. The filter system used to remove all dissolved solids from water is called an "RO/DI" system, which stands for "Reverse Osmosis/De-Ionized". This type of filter uses several techniques to produce very clean water.
If you go this route, you will also need containers and a few other things to mix the salt which we'll discuss later.
It's an odd thing to say to "make water" but that's what folks call it when they have their own RO/DI system that produces clean water.
This may seem like a very big endeavor, but many vendors sell systems that are ready to go. You just need to find a place to install it and provide source water. Later on, as parts of the system need replacement, you can get these parts from them as well. All in all, it is not terribly expensive, keeping in mind that you will need to make water for water changes fairly often.
You can start by browsing through the products offered by Bulk Reef Supply. Their products are of high quality as well as reasonably priced and they offer very good support to their customers. They also make great videos about this subject.
In my case, I have an existing RO system that feeds a faucet in our kitchen sink. We use it for drinking water as well as ice. When I started reefing, I added a DI stage so that I could have zero TDS water. Later on, I upgraded the RO system to one with a booster pump which produces less waster water and lets me filter more water more quickly. At the end of the DI stage, I added a long hose with a valve, so I can fill buckets of water more easily.
All in all, this is a somewhat complicated subject, but the key is to have a supply of very clean water.
Let's talk about salt next.