Don't stress it. Everyone on here is very helpful and just ask what you need, when you need. You have a gorgeous tank and once you get the hang of it it's a cake walk. Def test your water. I bought the API salt water test kit as well from a petco near me it was like $25. Meanwhile you dont have to keep bringing the water in to your store.
is a machine that you hook up to your sink or hose and it filters all the bad stuff out of the regular drinking water that doesnt bother us but can hurt the fish and build up in your tank so in the long run, its bad news. If you buy the water from the store, with bi-weekly water changes that can add up $ over time. I know petco near me sells boxed saltwater all levels and stuff are perfect but its $12 for 5G. it adds up after a while. An ro
is 3 cylinders the water flows through each cylinder and then drops out in to a bucket or whatever you want to use. This takes time, but its worth it. Once the water is in the bucket you can buy salt from your local fish store. read the directions and buy a hyrometer ($15 maybe) that you scoop the water with and it tells you the level of salt in the water. make sure it matches what's in the tank and mix it up.
That's all i really have for now take everyone elses advice and youll be aight!