theyre a little harder to take care of but Ribbon Eels are BEAUTIFUL and they could live in a 55g for life, personally their the most beautiful eel out there
Im not expert on salt water as i only keep freshwater but would invertebrates such as Blood shrimp? Blood shrimp (Lysmata Debelius) have an amazing color and would look really nice with clown fish. Also you could try Pacific cleaner shrimp (Lysmata Amboinensis) which also have beautiful coloring.