You can do it, "hard" is a relative description. If the 12g is heavily stocked with SPS, you need a small bioload (just saves you headaches and worries), and stay vigilant regarding the water parameters, in particular calcium and alkalinity.
My tank consumes around 20ppm of calcium and almost 1 meq/L of alkalinity per day (consumption rates are in proportion). So you just need to work out the demands, and dose accordingly every day.