I think the logic is:
Case 1: if the Aro swims sideways in just one direction (i.e. say the right side of the Aro is always facing upwards regardless of which direction he is swimming in): There is something wrong with one of the eyes of the Arowana.
Case 2: If the Aro DOES NOT swim sideways in just one direction (i.e. say the right side is facing upwards when swimming in one direction..when swimming in opposite direction, the left side of the Aro is facing upwards): There is a bright light source in the room, brighter than light in the tank. The Aro is using that external light source as reference and swimming (or trying to swim) perpendicular to that external light source. Hence, sideways to the normal direction
@Brian, please correct me if I'm wrong..
Good luck to OP,
Sid