The red and blue fish in Mark's aquarium are Betta fish, or Siamese Fighting fish. They are separated by reflective glass, which is necessary to keep them from fighting. Further symbolism of the severance of Mark's life.
Mark has a red and blue fish in his tank, reminiscent of the red and blue pills in The Matrix (1999).
At some point when Irving (John Turturro) is walking in the hallway (around 30:10), he talks about "Ambrose" being the black sheep of the family. This may be a reference to his character Ambrose on Monk (2002) being a recluse and misunderstood. It is a slight nod to his other character.