I would recomend going bigger than 50 watts. 50 would normally be fine. But, you mention poor heating in the building. Most heater calculations are for room temp or close. When it gets real cold here, we switch over to wood heat. This leaves my basement/tank room a little cool. A 50 watt in a 10 gal would strugle at a room temp of 60F.
It's like the lady said.(size matters)