Wasn't it ME that brought over the probe?!?
As for the temp swing, that is odd. I had MH on my shallow cube (which you would think would fluctuate easily) with the controller keeping track of temp, and the controller kept the tank between 26.0 - 26.5C. Never needed a fan to keep temps in check.
Being that your tank is in the basement, MUCH larger water volume, external return pump.. etc etc (see where I'm going with this?
) What do you have the actual heater set at?