If you garage really stays 60'F all winter then you are heating it and you just have to boost the water temp to the desired level. It is all in the details. It also help if your sump and tanks are acrylic. Plastic conducts the heat/cold considerably less. It is my experience that for every 2 watts acrylic needs you could need as much as 4 watts for glass. Heat rises so keep the sump covered as much as possible and still let the gasses out. A simple piece of Styrofoam works wonders.
