The Hydors are incredibe. I really like them. I have the 300 on my 75G with XP3 and it keeps the tank at a very stable temp, almost no fluctuations. Stays within a degree. Also, the 201 is the 5/8" model, and the 200 is the 1/2" model. I'm running the 201 on my 29G with the XP2, and works great. Probably would do just fine on a 20G as well.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006