Complex sentence: Mats of "Pistia" block gas exchange at the air-water interface, this reduces the oxygen in the water, killing fish, they also block light killing native submerged plants, and alter immersed plant communities by crushing them.
Split into simpler sentences: Mats of "Pistia" block gas exchange at the air-water interface, reducing the oxygen in the water and killing fish. They also block light, killing native submerged plants, and alter immersed plant communities by crushing them.