Turn on the Hoses!
Ok, it is time. I know you have been waiting along time to get here, but your patience will begin to pay off soon. Ideally, every time you filled up a tank you would use R/O Water. This is water that has been stripped of impurities through a process called Reverse Osmosis. You can usually purchase this from your local fish store, but this would be too costly and unnecessary for your initial fill up on a large system. Why is your tap water bad? Well, I can't tell you if yours is "bad", but I can tell you that it contains a lot of Chlorine. Chlorine is fine for us, but your aquarium inhabitants don't care for it at all. No worries. You can buy an inexpensive product that will neutralize the chemicals in your water. The other concern is how high the phosphates are in your tap water. This can lead to algae problems later, if you are constantly using water full of phosphates. I don't want to bog you done right now about the different concerns you should have about the water you use, but I just want you to keep that in mind for the future. But for now, turn on the hose!
So, now you are filling up your tank. Don't blast the water into the aquarium. Instead, let it run down the side glass or over rocks. Going slower will kick up less debris and allow you to watch all of your fittings and seals for one last leak check. If you have overflows, make sure that your sump begins to take on water once your aquarium is filled. You will have to start a siphon in your hang-on overflow boxes, if you have them. Just follow the directions that came with them. It's pretty easy.
Ok, now that you have your tank filled, be sure to confirm that your sump or canister filters do have water in them. You never want to start a pump dry. This can cause damage and lessen the life of your pump. Everything look good? Go ahead and turn on the pumps. Check your water levels and add more water if needed. Add your water conditioner as the directions indicate, remembering to add in the volume your sump contains to your total gallons calculation. Place your heater in your tank or sump and begin heating the water up to 77-79 degrees.
You are actually ready to add salt now. Refer to the given directions that came with the salt that you purchased, but the best practice is to go slow. Add a bit at a time, let it churn and dissolve. Now it is time to break out that new tool you bought, the Hydrometer. If it does not have directions with it don't worry. All you do is submerge the Hydrometer into the water and fill to the indicated level. Set it on a flat surface and check where the needle indicates. Be sure to tap the side of the Hydrometer lightly. This dislodges any air bubbles stuck to the needle, which will throw off the results. Continue to add salt slowly, given ample time for salt to dissolve, check your hydrometer and stop once you have reached the level of 1.020 - 1.024, as indicated on the Hydrometer. Once this level is reached stop adding salt. You now have a saltwater aquarium! Well, you're a lot closer anyways, but there is still work to be done.
It is now time for a quick lesson in pH. In a simplified definition, pH is the measurement of the concentration of Hydrogen atoms, H+. What does that mean to aquarium keepers? Well, to us it helps us determine the Acidity or Alkalinity of our water. The pH scale runs from 0, most acidic, to 14, highest alkalinity, where 7 is neutral. Typically aquarium fish require water in the range of 5 - 8. Marine fish come from a pH range of 8.1 - 8.4. So, what the pH buffer does is helps to maintain that higher end range. Now is the time to add the pH buffer to your system according to the instructions for the brand you purchased. You may not register in the correct range right away. Like the salt, keep adjusting over a period of time allowing the water to churn and mix.
Now adjust your returns and circulation pumps so that there is some movement in all areas of your tank. Try to eliminate as many "dead spots" as you can. This is important for the next phase.