A baby tang maybe? Man, there are some of the most ADORABLE little baby sailfins here at a LFS not much bigger than a toonie. I just about picked one up, but, I've decided to wait because they're Z. veliferum (as gorgeous as that fish is), but what I really want is a Z. desjardinii, and I know I can't do both.
I don't know if the 15g is a universally accepted threshold. It's just a subjective thing. "Nano" seems to imply "very small". I consider my 20g reef a "small" setup but not a "very small" setup. So I dunno ... I guess that doesn't qualify me as a "nano keeper" by my own definition but here I am posting anyways.
Whatever terminology works for you.
BTW, I have a sump on my 20g. And if I ever set up my 5g as a reef, I was thinking of putting a whomping big sump on that thing. Why not? A sump to me is the essence of simplicity. Hide my heater, hide my skimmer (if I choose to use one), and best of all (everything else is secondary, really, to me anyways) WATER TOP UP!! The system can withstand a little evaporation and the water level in the main display doesn't fluctuate. (Float valve!! Heh heh ... is there really any greater labour saving device for reefing?) Not that it's necessary, but that's really why I like sumps right there. I'd put a sump on my freshwater tank if I was to ever take it down and set it up again.
Anyways just my two pennies worth ...