Depending on the range of your thermometer, a mercury thermometer can be calibrated by sticking it in boiling water where the temperature should be 212F or 100 C.
In ice water with crushed ice and little water the temperature should read 32F or 0 C.
But often a mercury thermometer can have a split in the mercury column. You have to check closely the whole column. If it has a split or gap, it is off on the high side by the amount of the gap. Sometimes you can repair these thermometers by placing them in dry ice which will suck all the mercury into the bulb. Then let it warm very slowly in an upright position and the gap should be gone.
But for the most part a difference of 2 or 3 degrees is not significant in the aquarium and will not likely have a noticeable affect on the fishes behavior.