I've heard them referred to as "fish cookers" many times!turbotiber said:Same dilemma (except instead of 20 I only have 6). They sell these little heaters here, 5watts and supposedly that's the rule of thumb right? 5 watts per 5 liters? What do you guys thing about those??

I wouldn't use a 25w heater for a 1g, but on tanks 2g or higher, I do use a higher quality 25w heater. Some people think it's a bad idea, but it's worked well for me. I just monitor the temp daily.
For anything under 2g, I'll either get a heat pad, or try out cluttery's awesome idea. This is my first winter with more then one betta!