I brought a 125 gallon saltwater tank. It came with a wet/dry filter, overflown to sump, 1500 GPH pump, power head and etc. It is very dirty and has salt and algea. I want to transfer my freshwater discuss, neon tetras, pleco into this tank. My concern is the discuss fish. Can they live in this system? Is the wet/dry filter appropriate for them? How do I go about cleaning the tank? Any advices are welcome.

My dad said tropical fish are saltwater and colwater fish are freshwater, but I disagree, I say tropical fish are fish that like plants and coldwater fish are fish that like clear spaces to swim….?
Addition Question: Are Tetras (Neon Tetras) Tropical or Coldwater fish?