tropical are freshwater heated to around 75 degrees

marine are saltwater reef sea fish also heated you need a tank with holes and a sump with filteration tanks below having filters inside the tank will be broke in a matter of weeks you also need some strong power heads for water flow other wise your corals will not be very happy or your fish you will need protein skimmers too they are expensive we had a 6x2x2 marine set up and it cost us over 3k to have it nice and fully functioning fish are also expensive and there is a lot of problems if you stock too fast or or dont have the correct water conditions there is a lot to sort with regards to regular water tests too i wouldnt reccomend a beginner keeping marine as we had fresh water tropicals for many years before setting up marine and have since gone back to fresh water and our current tank is 52"x20"x18". i prefer the natural freshwater tanks to marine as you have major algae blooms in marine tanks especially in the first month or 2 then you need the algae to keep the tank running ok and its unsightly