Any suggestions as how to decorate 30 gallon tank ??? ?

75 gallon 125 gallon come on unless your rich and have lots of extra money to spend not to mention the space for the tanks then that's your best answer. If you don't have all that then i say that a 55 gallon will be plenty of room for two adult oscars. I don't think you intend to buy them at that size so i would just buy a couple of small ones put them in your twenty gallon and watch them and decide if you really want to spend that much money on tanks or not. you will have plenty of time to decide as they don't grow to adulthood over nite.