LOL I have 3 different aquarium thermometers 1 battery operated one, and 2 that hang on the side, ALL 3 read different temp one says 80, 1 says 82 and 1 say 78, so how do I know which one is the right one?
Digitals tend to be more accurate, but where they are placed, side of tank, light facing glass, depth, etc., etc., etc., all could very easily cause a 2-3 dgree change or more.
From what I recall, they say that your thermometer should be at the other extreme of the tank. My heater is at the top left side of my tank, so I put my "stickered" thermometer on the bottom left. I also have another glass thermometer that is on the top right side of the tank. The difference is about 2 degrees from the bottom of the tank to the top. The top thermometer reads higher as heat rises.