ooh add some gobies I think they look great in nano aquariums.
I would suggest the common blue stripe neon goby or if you wanted a less common fish you could do a yellow stripe neon goby.
black or green gobies exept they are not very active but green clown gobies are colorful.
you could also do bangaii cardinalfish, I wouldnt recommend it in your size aquarium but you could try pajama cardinallfish
bicolor chromis, scissortail damselfish, and some more pricey but cool fish are yellow assessors, or blue assessors I would want the yellow assessor because of the extra color. Another good coice is blennies but they do seem to get larger and would not be the best choice you your aquarium
hope I could help :)