You could add 2 Angelfish or a pair of trio of Pearl Gourami's. I think I'd go with the Pearl's. For the bottom level you could do 6 Cory Cats or 3 loaches like Yo-yo's or Angelicus. Or instead of larger loaches you could do a group of Dwarf Chain Loaches or Kuhli Loaches. Then you could do a school or two of smaller fish like Neon, Cardinal, Rummynose, Lemon, or Kerri Tetra's. Threadfin Rainbowfish, Marble Hatchets, Purple or regular Harlequin Rasbora's are all good choices as well.