I have a pair of Hawaiian leaf fish in a 55. They get about 4 inches long, mine are a bit smaller. My leaf fish will only take live food - shrimp, guppies, gambusia. I think a 12 gallon nano is on the small side for most scorps. If you can, I'd get a 24 gallon nano cube or better yet, use at least a 29.
In my 55, I have several scorpionfish and relatives. They are all ambush predators, some take only live food, others will take frozen:
-2 Hawaiian leaf fish - live food only
-longspine waspfis h - frozen and live - I have a mate for this one currently in another tank
looks like this:
-orange banded stingfish - just got it, hear they are hard to wean to live - here's a pic of one similar to mine, mine is less colorful:
This tank is pretty packed right now. I have other fish now in the 55 that I'll have to move soon:
-small bluethroat trigger - going to the 210 when it is set up
-small green wolf eel blenny - will need to eventually move, maybe to the 210 - very cool fish
-3.5-4" yellow angler, Antennarius pictus - will go in my 29 reef when the angler already there gets a little bigger
The leaf fish seem to be thriving, I've had them for quite some time. This tank has lots of live rock (65-75 lbs) some hardy corals, lots of different macroalgae. Shallow sand bed, fairly low flow. Filtration is BakPak skimmer, Emperor 280 HOT power filter and an Aquafuge HOT refugium full of macro and lit 24/7.
I hope this helps you some. I don't know many sources of good aquarium care information for leaf fish or other scorpionfish. Scott Michael's book Reef Fishes volume 1 is about the best source I can recommend.