In a 55g tank you have a lot of fish options. You can look into these species to see if any interest you. Threadfin Rainbows, Dwarf Neon Rainbows, all the many colored Rainbowfish, Neon, Cardinal, or Rummynose Tetra's, Diamond tetra's (one of the larger peaceful tetra's), Harlequin or Purple Rasbora's, Kerri Tetra's, and many others. Then you can look into Cory cats or many species of Loaches. Look at many different types of fish and write down some combinations you like and then you can run your idea's by everyone. Rainbowfish have an incredible array of colors to choose from.