I'd skip a mandarin. Especially as it looks like you don't have a Refugium setup to grow pods. Honestly they really are a fish best left in the ocean. If you must do one look into the captive bred ones, I believe biota breeds them.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"