The water hyacinth is probably the world's fastest growing surface plant and within weeks covers the whole surface and block the sunlight to other plant life below the surface, not only killing the plants but also the animals that feed on it. The lack of oxygen in the water below is also very detrimental to other aquatic life.
Originally native to the Northern parts of South America where it was controlled by manatees and the ecosystem, it has been introduced to most parts of the world with disastrous results.