The spill was caused by a corroded pipe that transported the molasses from storage thanks to ships. NPR says 233,000 gallons of molasses were spilled into the harbor on Monday before the leak was stopped on Tuesday. That's enough to fill seven rail cars, FoxNews said.
You can kind of see the outline of the brown stain in the water in this aerial shot from NBC news:
Hawaii News Now - KGMB
A little sugary liquid doesn't seem like it would destroy an entire community of animals, but because the liquid is heavier than the seawater, it sinks to the bottom and disperses far and wide. Because it doesn't carry oxygen as well as water does, the sea life suffocates.
Hawaii News Now - KGMB
Hawaii News Now - KGMB