stealth is an excellent heater. I also have a 300 gal, and I have 2 visi-therm 300w and 1 top fin 300w. I guess some has to do with the temps that frequent your area and house insulation etc. I would recommend starting with 2 stealth 250w(highest watt stealth heater I've ever seen). If they stay on alot then I would get a third one. The visi-therm is nice also and a little cheaper. Thats my 2 cents.