Winston is great about looking through glass. He's a total couch potato who will watch the entire 10 o'clock news with you. So, I am not surprised that he eventually found the fish tank had critters in it that would interact with him if he got close enough.
The guppies only swam closer to him because they don't know the difference between one object moving in front on the tank from another. To them, any moving object in front of the tank means FOOD. Which is great, because it makes it easier to photograph them (but at the same time it doesn't help they are the quickest fish ever).
The dwarf gouramis stayed curiously nearby, until he started licking the glass xD now THAT was funny.
But as for my blue gouramis and my betta, they were smart enough to hide from him because he was slightly a threat, especially when he got a bit too playful.