Three things not mentioned on the board...*SPOILER*
1) the koi fish are a symbol of love, friendship, and prosperity. They were a gift made in that generous spirit. When Simon (unknowingly being recorded) behaved like his old cruel bullying self, and cut the evening short with Gordo by telling him to stay away (after making sure his wife didn't hear the reason why), Gordo killed the fish. They were a gift. And the gift was rescinded. Amazing symbolism, I had to look up 'koi fish' because I knew it had meaning and I just didn't know what.
2) Gordo was discharged from the military (where Gordo got his surveillance training) for "Conduct Unbecoming." That is code for 'gay', and the earring on a grown man in his 40s in Los Angeles is pretty gay. That is why Robyn, upon meeting him without knowing any of his history, called Gordo 'socially awkward' and knew Gordo wasn't trying to bang her (despite what Simon claimed). The gay thing is also why Gordo's father set him on fire, and Gordo was an easy and believable target for claiming he was being molested by an upper-classman. Gordo did not rape Robyn (shown in the DVD Extras), and as a straight guy I can say with great authority that the ONLY reason not to take advantage of an unconscious Rebecca Hall is because you are gay.
3) Simon was 'gaslighting' https://en.wikipedia.org/wiki/Gaslighting his wife Robin, by trying to persuade her that Simon wanted to bang her when he knew full-well Gordo was gay. He was trying to turn her opinion, just like he did to the kids in high-school 25 years earlier with his bullying and lying. Marking 'Weirdo' over the 'Gordo' phone number, mocking Gordo with the silly voice and his finger through his zipper when Gordo left the house, and telling a false story about Gordo's intentions, which Robin intuitively knew that Gordo wasn't after that. Robin had suspicions all along, hiding around the corner to hear what Simon was going to say to Gordo when Simon told Gordo to stay away, looking into Simon's desk drawer to find those private investigator profiles on Simon's "victims", etc. Ugh! Total sociopath!
Amazing script by Joel Edgerton. If you liked it, you will also like "The Rover" starring Guy Pearce which Edgerton also wrote.