Silver dollars really aren't suitable for a 55g but you could do a colorful Rainbow fish tank, a community tank with a couple of angels, a dwarf gourami, a school of tetras such as cardinals, rummynose, lemons, or diamonds. Then add 6 cory cats or 2 yo-yo loaches and maybe even a school of hatchetfish or harliquen rasboras to use the upper water level. You have alot of options.