Never return fish to the sea. They likely come from some other ocean and can kill local species. Lionfish entered the Caribbean from that sort of action, and are now a major problem.
That said, first aid for your situation, which is almost certainly lack of oxygen, is an immediate 30% water change, a carbon bag in the water flow, removal of surviving livestock to clean new water in a qt tank, and keeping them there until you have corrected the situation.
No matter where you live, you can purchase tests, salt, appropriate pumps (yours may be too small) live rock and sand, etc, for any tank, including tanks---online. Check our sponsors list. These are reputable merchants. You need a qt tank, at very least. If your water is green, you have a phosphate issue, and should be using a ro/di filter.
We're glad to help, but the situation you describe is a tank crash, and you are going to have to get that surviving fish out to another absolutely clean bare glass tank with good aeration asap, while you take measures to rebuild your tank ecosystem.