DES MOINES (AP).- A new exhibit at an aquarium in Iowa that had intended to showcase the beauty of the Gulf of Mexico will instead be void of life to underline the environmental impact of a massive oil spill in the ocean basin. The 40,000-gallon aquarium at the National Mississippi River and Aquarium in Dubuque, about 1,000 miles from where the river dumps into the Gulf, was supposed to have been teeming with sharks, rays and other fish. Two smaller tanks were to show a seagrass bed and coral reef. “It may be the only time that people have ever seen a major aquarium that, instead of showing its fish, is showing an environmental disaster,” said Jerry Enzler, the museum’s executive director. The main tank — the size of a school bus — will contain water and artificial coral,