Thermometer Accuracy

[stanleo]

I have a Coralife Digital Thermometer. I wanted to lower my temp in the tank and I couldn't do it with the old heater so I bought a new one that was easier to adjust and read. I took out the old heater at 3:30PM yesterday. It is now 12:30PM today. In that time the thermometer went from 80.4F to 77F. The lights have been off since midnight. The apartment is 72F. How long should it take for a 55gl tank to reach room temp? I tried to test the thermometer with ice water but it only got down 34.2F. Do you think I should get a new one? If so, which one should I get?
 
My target temp is 77F. Should I go ahead and turn on the heater set at 76F? I am thinking about not using the heater and seeing what the lights do to the temp. It might be healthier for the fish to have a lower temp at night. What do you all think?

 

[Ch4rlie]

I have two types of thermometers in my tanks, one glass one, cheap from online or LFS, and a digital one from online.
 
These thermometers are in tank at opposite ends to ensure tank temp is even throughout the tank.
 
A bit much for most keepers as usually just one thermometer is usually adequate but thats how i like it set up
 
BTW the stick on thermometers are not all that good, so I'd say to avoid these.

 

[Mamashack]

The thermometer is probably right if it said 34.2F as that is only just above freezing - it would only be 1 C.

 

[Fishmanic]

I heard that stick on thermometers are more accurate on glass rather than plexi tanks.   I find stick on thermometers to be very close to the temperature I read on a glass thermometer that I also keep in my tanks.   I use the stick on thermometer to get a quick read of approximate temp from a distance so I can be sure the heater is working ok.  
 
I don't have any experience with electronic digital thermometers so can't comment on those except that I've read good and bad reviews for electronic digital thermometers as far as accuracy goes.  As far as the guage setter on heaters go, I wouldn't rely on them to be very accurate.  I find the best thermometer for accuracy is a good glass alcohol thermometer.

 

[Mamashack]

I used to have a stick-on one on my 30L glass tank and when I got a digital one it was several degrees different (and that was backed up with a glass one on suction cup).

 

[stanleo]

I bought a glass one and it is reading 77F while the digital is reading 79F. I think the glass one is more accurate. 

 

[Mamashack]

I think you are right, stanleo and the glass ones more accurate tho perhaps slower to respond to changes
In answer to your original question I think fish do respond well to the diurnal changes in temperature so 76 is probably a good place to set the heater if you're aiming for 77 - what is the suggested temp range for your fish?