Not sure where in CA
you are, and what exactly "pretty cold" means to you! I guess the obvious answer is get a 100W heater and see what happens.
I've just noticed the same thing with my saltwater mixing tub in my garage up here in WA state. I've had to switch from a 50W heater to a 100W to keep the temp around 76 or so. With the garage in the mid-high 50s, a 50W heater just wasn't able to pump out enough heat.
One thing that might help is just throw a blanket or towel around most of the tank. That will at least make a little micro climate around your tank and provide some small bit of insulation. In addition sit the aquarium on something insulative. Styrofoam, cardboard, anything other than the concrete floor.