I suspect lack of light. How much light is on the tank?
As an aside, this is a little article I've written on it in the past.
Common Name: Water Hyacinth
Scientific Name: Eichhornia crassipes
Light: High to Very High
Co2: Not essential to growth
Level of Difficulty: Medium
This pan tropic plant is a floating plant that develops large, dark green, and glossy leaves.It is famous for its beautiful lavendar/purple flowers which grow on stalks atop a large stem. It is now found throughout the United States, particularly the Southeast, though portions of the West and Northeast now contain this plant. It is considered a "noxious weed," "invasive species" and a "plant pest".
In the aquarium, it is suited for open top tanks, otherwise the plant will not have room to grow. It provides a dense, lush floating canopy, offering shade and refuge to light shy or skittish fish. Gouramis and anabantoid species appreciate the hanging root structure. It blocks significant amounts of light, however, and is not suited for tanks which must have a lot of light for the plants below. When adequately housed, it makes for a stunning appearance and is a wonderful alternative to the more common floating plant species.
In aquariums with clean water and adequate light, it reportedly maintains a smaller size.
In the wild, this plant has been responsible for clogging waterways, pushing out native species, and altering entire ecosytems, and has plagued Florida for over 100 years. Make sure that this plant is disposed of only in ways which do not put native waterways at risk.