Which thermometer is accurate?

Ok, I have a 10 gallon tank and I bought three aquarium heaters and temperature. always was 60F. None heaters keep water at 75F. I bought my heater session (Visi-Therm) and a digital thermometer that sticks to the glass. Waterproof Thermometer said regular water is 62F and digital says water is 82F. I put the heater in 83F, but I want to make the water is at a safe temperature. for my Betta fish. What thermometer is accurate?

Something is very wrong if a heater fails to heat a 10 gallon tank above 60 degrees F. Are you sure that the heater was connected and on? If the heaters of three or four different, they all gave the same result, I suspect the thermometer is off. Thermometers sold for aquarium use are notoriously inaccurate. When I go through a stack of thermometers in the pet store, often shows all the different temperatures. It is always a good idea to check against one another thermometer that is known to be necessary (which does not have to be an aquarium thermometer.) The "label" type of digital is not necessarily themometer more or less accurate than more conventional types, but I do not like the type of label, as it measures the temperature outside of the cup, no water inside the tank, and they can be different for the long haul.

10 gallon Freshwater Aquarium