First of all, are you suggesting that Romney is somehow corrupt (i.e. your suggestion that he has his money in Aruba)? If this is the case it is clear that you would say that ANY Republican candidate was corrupt — and it completely undercuts any position you might advocate.
Second, there is absolutely no requirement that Trump make his tax returns public. In fact, in the United States everyone is guaranteed the right to keep their tax returns 100% private. With more than 500 companies and 12,000 pages of returns revealing the content of his returns might negatively impact his business operations and provide fodder for his opponents. If he were my client I’d recommend that he NOT release them.
Over the past 40 years the tradition of releasing returns has been for politicians — most everyone running for president has been a career politician and the people wanted to know if they were using their office to make money (i.e. Hillary Clinton). With someone like Trump — who has never held political office — and his returns have been audited for decades — it is clear he hasn’t been getting political payoffs or cheating on his taxes.