The extra plant life adds plenty of oxygen to the water, which is a good thing, but eventually those extra plants die, and aerobic (oxygen demanding) bacteria decompose them, consuming oxygen in the process.
(The extra plant life; adds; plenty of oxygen; to the water) (the water; is; a good thing) (those extra plants; die; T:eventually)
The extra plant life the water those extra plants
adds is die
plenty of oxygen to the water
a good thing
T:eventually