. I would advise you to stay away from the "hang on" heaters. They are not as accurate and are generally inferior to the submersibles. Visi Therms are good heaters. I have used them and have had very good luck with them. Ebo Jager is also a good unit. 75w is about the bare minimum for a 20g. I would go with two 50w submersibles. This gives you a better margin should one of them fail. If one quits heating, the other can take up the slack reasonably well. If one hangs in the "on" position, the lower wattage will give you more time before you have boiled fish. Also, never trust the temp scale on the heater. Get a good thermometer and use it to set your heater. The temp scale on the heater will rarely be exact and I have seen them off by 6-7 degrees.
