Yea your basically torn between a snowlfake and Golden Dwarf (which are rare and expensive). Anything else would get waaaay to big.
There are other gnarly things you can stick in a smaller FO tank like that:
- Small Puffers
- Clowns
- Damsels (Though I'd always recommend a school of chromis over anything else. They show no aggression unlike their mean cousins and are fun to watch)
- Midwater gobies like Firefish are always a great choice as long as you have more peaceful tankates and a lid.
- Dwarf Angels
- Gobies and pistol shrimps are a blast to watch
- Lionfish
- Other FO wrasses (banana Wrasse for example)
- Dont quote me on this, but you may be able to pull off one of the less aggressive triggers like a niger or blue throat. Also if you added a trigger, he'd basically be king of the tank and there isnt much else you could add