The water they were in was much warmer, and held much less oxygen, so the fish were gulping in more water and trying to move more water over their gills to try and get more oxygen.
(The water; were; in) (The water they were in; was; much warmer) (The water they were in; held; much less oxygen) (the fish; trying; to move more water over their gills to try and get more oxygen)
The water The water they were in The water they were in the fish
were was held trying
in
much warmer
much less oxygen
to move more water over their gills to try and get more oxygen