For a 20 g you probably don't need one (with an ice probe anyway)- I don't have one for my 16. 20g is pushing it for the ice probe to do more than slowly keep the temp 1-3 degrees cooler. It will probably take a full day for it to have a noticable effect.
I just have my heater set at 76- if the tank drops it comes on and more than compensates for the ice probe. Probably uses more electricity though...
well the heater kicks on if the tank temp ever goes below 76...even w/ the ice probe the heater has no problem bringing the temp up again- the ice probe really isn't all that strong of a chiller and won't bring down the temp more than a few degrees in anything larger than 15g...nothing a heater couldn't keep in check- and in the summer it rarely gets down to 76 in my tank anyway, with the chiller on. So the tank temp stays stable
due to the fact that the ice probe really takes hours and hours to make a noticable differance in a larger tank, a controller turning it on and off won't allow to it actually do its job as well ans it can without one imho...