I have the temp set at 80* in my office (where I keep my fish) and it rarely gets 80* in here (probably from people opening doors to the outside though) but my individual tanks refuse to stay anything warmer then 70* during the day...mine prefer 80* or they mope around lol...so I'd hate to think what it gets like around 12-1am around here. (and the codlest it's gotten here lately is 18* at night..usually hangs around 22-28*)
Guess it would really depend on your fish, how warm the water stays in the jar throughout the day, and what your fish prefer and can handle.
Also, if you use the heating pad trick...make sure you take the cover off it...then put the cover under the pad...that way it doesn't have any tendency to scorch whatever it is sitting on. (Make it like a pot holder) It is also a good idea to sit it on a baking pan if you can...for added protection.
There are other options avaliable though if you or your parents (I have no idea how old you are) are afraid of fire hazard.
There are these lights made for reptiles that even though they burn at a dimly lit capacity..are made to produce heat...so you can leave them on all day long..and they shouldn't produce algea.
Reptile Black Night heat lamps.
This bulb provides the perfect heat source that allows your reptile to sleep at night in comfort at the proper temperature for its night time requirements. The special coating on the bulb provides a natural night-light simulating moonlight which does not disturb your reptiles sleeping patterns.
* True black glass (not painted or coated) for better heat transfer
* Ideal 24 hour heat source for all types of reptiles, amphibians, birds or small animals.
* Very little visible light provided so your animal's sleeping patterns are not disturbed.
* Perfect "night-light" bilb for viewing the nocturnal behavior of your animals.
* Perfect for use in horizontal facing light hoods
* European quality for long bulb life.
Available in 50, 75, 100 watt
You can also use under-tank heaters..and sit them on those.
Or a reptile heating pad...same as a regular heating pad..only made to be used that way (little more expensive and harder to find..at least around here).
You can, however, find all of those items for a discounted price on Ebay..just type in the description and numerous items will pop up.