50 is nothing, just put an extra heater in. winters here in WA cause my garage to go into the low 40's, without the tanks in there even below freezing. i just run 5watts per gallon and cover the tank with blankets, all my tanks were glass last winter and not once did the temp drop below what i had them set at.
now a 400g tank will be 2000w, but with the blankets they really dont run that much. im shure i couldve run less heaters, but their cheap insurance. you could run 1000w non stop and worry, or run 2000w and not, either way it takes the same amount of energy to heat a body of water.