Interesting one I got advised to look at or 55 gallon is dwarf lionfish if you don't mind the live food feeding
A Dwarf lion is a great addition to a tank 50gallons and up.
Although I do disagree with having to do live feedings, they can and will (after a small period of time) eat frozen, pellet and even flake foods. It will be easiest to convince them to eat frozen more than any other food, as items like frozen bloodworms have those good nutritional juices still inside the cube when thawed out. You can convince a Dwarf Lion to eat by the following ways; you can keep on feeding frozen food and let him/her starve until they decide to eat it (I know it sounds barbaric and unorthadox, but it works and they should turn out a happy little lion)
OR
You can feed them live ghost shrimp and keep on introoducing more and more frozen foods into their diet and they will gradually accept it.
*NOTE this method may take weeks to months to ween your lionfish, but it is a good reliable method.
I also suggest a fuzzy dwarf lion, as they are the easiest IMO. If you want a much better looking lion, IMO, choose the antennata lion, they have a more striking colour and their poisonous spines are longer and more prominent than other species of dwarf lions.
Anyway the fish I would suggest is a Golden Dwarf Moray Eel, or a snowflake eel. Golden dwarfs do look very nice and stay a much more manageable size at a mere 1' and are considered very peaceful compared to other morays. The snowflake would be a much better choice if you're looking into a predator tank, they grow to around 2' but will still do well in a 55, given they have plenty of LR and a protein skimmer would make life easier, because they are messy eaters they pollute the water, hence the need for a protein skimmer, as it would enable you to do a smaller water change, less frequently. Depending on your stocking list
. say this was your stocking list:
snowflake
antennata lion
and a pair of maroon clowns.
around 15% bi-weekly would be adequate to keep water parameters in check.
Hope I helped.
- Chris