ROME (Reuters) - Experts are scratching their heads in concern and confusion over what has happened to Rome's Tiber river where tons of dead fish have floated to the surface and algae have spread like the plague. Environmentalists say the phenomenon may have wiped out two-thirds of the fauna in a five-kilometer (three-mile) stretch of the river that runs through the heart of the city. Tons of dead fish have floated to the surface since July 15, leaving a stench hanging over the city center. Even eels, the Tiber's hardiest denizens, have leapt onto the banks to escape the water....