There are many brands of 100w heater that would do for that 20g. Ebo-Jager, Tronic, Hagen's Thermal line all make 50w and 100w. At the going rate of 1w per 5g, you're looking at a 100w heater.
Don't go cheap with heaters, and try and get a submsersible one as they tend to be better made.
