I can be across the room, and they will follow my movements through the tank, watching me.
When the dogs are playing with their toys and running from one end of the room to the other, they frequently follow them from one end of their tank to the other.
And when I approach the tank, they almost always begin to look to the surface of the water for their food. When angelfish are watching closely, they will often move from side to side. With an eye on each side of their heads (versus ours, which both face forward) they can see almost 360 degrees. Their only blind spot is directly behind them but their peripheral vision is far better than ours.