It differs in your location and what you normally keep your house temp at, but, I used 300w's on my 120 gal tank and now use 600w's on my 225 gal tank. Neither one had an issue with raising the temp in under and hour. I keep my house at 64 degrees during the winter, so yea, pretty cool. I would just use a controller to have some safety in the heater as it probably is the most common failure. You said that your total water volume will be roughly 55 gals, you have to remember that rock and sand subtracts from the total water volume. I currently have a 225 gal tank and roughly a 60 gal sump. My actual total water volume is still 225 gals. I also do not like to keep a single heater. I break them up and use pairs of heaters for further safety. I currently use 2 300w BRS titanium heaters on a BRS controller with my GHL as a backup.