Hi. I have a 55 gallon tank that is 24" tall, 48" wide, and 12" deep. I have an Emperor 400 that seemingly creates a fair amount of current. It's place in the back left position. My heater sits to the right of the filter. It's a Marineland 200 watt. I have two florescent lamps which run the width of the tank; both on. My room temperature is 61 degrees. (It's Minnesota, which brings up another question for another time.)

Initially I placed my in tank thermometer in the corner farthest from the heater, front right. Today I added two thermometers and put one in the front left, and on fairly close to the heater. All three have their bulb 6" from the surface. My temperatures in the tank range from 65-69, and my three 4 to 5" goldfish spend most of their time in the front right, the coldest place in their universe.

So, I began to wonder. What difference in ranges are usual in a tank's geography? Is this a more interesting situation in a much larger tank?    

This is not a question of "temperature ranges" but as simple as "don't put the thermometer by the heater" That shows an unaccurate temperature reading since the water obviously is warmer close to the source of warmth

It's very important that you keep the heater close to the streams of the filter to make the warm water spread better in the tank to keep a more even overall temperature It's logically harder to get good circulation in bigger tanks, and so, this kind of "temperature zones" will appear. An interesting topic of discussion though