Question: Two questions about the scene where Dan is talking to Larry online and pretending to be Anna. First, why did he want to set her up with someone if he wanted her? Also, how could he be sure that she would be at the aquarium at a certain time? I know that he knew she liked to go there, but how could he be certain that whatever guy he played the trick on would meet her there?
Answer: I think he set her up because he wanted to prank her for rejecting him. Just an unlucky fluke for the second part with them meeting up at the same time, I think. Or maybe 1pm is when she takes a break or gets off work at that time? Who knows. It happened, though.
Answer: My theory is that Dan was trying to create a bunch of sleazy, annoying encounters for Anna to deal with. As another comment says, he probably did this several times, knowing that she liked the aquarium. He thought she would finally give him a chance after dealing with those men from the chat rooms. But it backfired—he actually helped her meet a future husband.
Answer: Dan did not know Anna would be there. This is revealed at the art exhibit when Anna tells Dan that she met Larry at the aquarium as well as their nickname for him - Cupid. He unwittingly created an obstacle for himself.
I think he had played the Internet prank/joke a few times before. He knew that Anna liked the aquarium, so if he chatted with multiple men on different days, there was a chance that Anna would actually be at the aquarium when a man arrived to meet her.