its easy to heat a tank, its hard to stop it over heating, 10 watts a gallon is a guide i have read many times, i have followed it, and remember when your up to temperature, it will switch off, so size isnt the be all and end all, when your aquarium is up to temperature, the heater should only flick on and off when heat demand is required, i do agree though analogies 300 watts for asmall tank is alot, but hey, it is not on all the time, and anyhow 2 heaters are probably better than 1, except in a small tank, perhaps in very small tanks 1x 150-200 watt heater will do but i still believe 10 watts per gallon is safe, anyhow, what is the difference in price between a150 watt and 300 watt heater and are they that much more massive in size