My water hyacinth in bloom.

The water hyacinth is considered an exotic plant, but is very invasive. It's actually illegal to transport or sell them in certain states in the US since they have invaded entire lakes. They grow quickly, and can double in population within 2 weeks. Ultimately, their invasiveness causes them to completely deplete the oxygen from the water and kill off all fish and aquatic life.

I have a couple in my fish tank, which I use to help filter nitrates. Here are some photos of their most recent flowers.