@Mom2Many Is there any that you recommend?I know you are great with Nano tanks
Thank you for your vote of confidence.

For what it is worth here is my 2 cents.
With what you have in there already a yasha would be great but they are shy and in my experiance are jumpers in small tanks. So unless you stalk them, you might not see them to much. Any type of firefish should go well in that sized tank and with what you have in there. Yellow or green gobies would get along and be fine in your sized tank. The yellows have the most personality. I second the harliquin serpent sea star. Mine is very visable because of the striking blk and white pattern. They never get big enough to cause any trouble. Mine is almost five years old and is only 8 inches accross. I got mine from KP aquatics and was three inches accross when I got him. It has been fun watching him grow. I also have a small fighing conch that is fun to watch. Now
MOST people would disagree with me... but I have had good luck with Springers Damsels and only springers damels...IF KEPT SINGLY. and if they have enough room to have a small bit of teritory. Now dont hate me if you get one and it eats all your other fish.

But when I have kept them singly, they seem to stay pretty passive. I have kept one in two seperate set ups with great success and that striking blue is impressive. I have one now in my 20L. I love him. Like a mini dory. (so get one at your own risk)
I have kept in 20Ls over the years.....clowns, six lines(hit or miss with personality) clown gobies, tailspot blennies, royal gramas (hit or miss with personality) manderians (constant headache to keep it fed), singly kept springers damsels, all kinds of shrimp gobies from watchman to antena...possum wrasse (very shy), firefish, very small hawk fish(cant remember the name)... serpernt seas stars that stay small enough, porcline crabs, pom pom crabs, anenome crabs, sexy shrimp, fire shrimp, cleaner shrimp, pepermint shrimp and the list goes on. We can be a bit limited in smaller systems.
I know that sounds like a lot of livestock I have owned...but I must admit, I am a reformed reef addict and have had many tanks over the years. Just a few years ago I had 13 tanks in my home both salt and fresh. So I have played around with many creatures, and ways to care for them as far as nano tanks go.
I know you are going to pick something great! I cant wait to see!