Fermentation Temps with rubbermaid and aquarium heater. .

I'm building my temperature controlled fermentation chamber (rubbermaid tub with a aquarium heater) and I was wondering . . if I put the primary into the tub and set the temp at 69F (This is a Wheat beer with a belgian yeast) will the temperature inside the fermenter (glass carboy) be the same as the temp of the water outside? I plan on filling the water on the outside up to about the 3 - 4 gallon mark of the carboy.

I ask because I want to keep my beers at a specific temperature! As close as possible at least, with the least amount of change.

I know a few of you out there currently use this method. . what say you?

Yes, generally the water is about the same temperature as the beer. If you add more water, the temperature will be even closer. That much water insulates the fermenter pretty well.