Complex sentence: Despite the large size adult alligator gars can attain as adults, they are kept as aquarium fish, although many fish labelled as "alligator gar" in the aquarium trade are actually smaller species.
Split into simpler sentences: Despite the large size alligator gars can attain, they are kept as aquarium fish. However, many fish labelled as "alligator gar" in the aquarium trade are actually smaller species.