Not that he should even think about dragonets at this point, but a 30 gallon with lots of rock and macro algae with a 20 gallon refugium can be sufficient for a smaller pair of dragonets. I actually used a tank system like that to fatten up mandarins.A pair of dragonets would be a poor choice for a tank under 150 gallons in my opinion unless you plan on buying pods on a regular basis for them. I doubt one would survive in your tank at this point. They eat pods all day long. It is amazing how many they eat.