If you are worried about the amount of heat up there and its effect on an aquarium, why not put a gallon jug of water up there for a few days and check its temp. on a regular basis. An accurate digital kitchen thermometer would do. Then you will know. Make sure the fire place is in use if that is how it is going to be normally.
Most mantles are only decorative now days. Some are only 1x10's with some minor bracing and decorative molding. Good for pictures of the family, not so good for things with lots of mass. That much of a load (tank size x 12 lbs/gal
. approx.) may break the mantle, the tank, or rip the whole mantle off the wall. Look into how the mantle is constructed and anchored. That will help you determine what it can safely handle. Maybe test it with sand bags or similar first. If you can't see it holding the weight of a person it won't hold even a small tank. (10gal tank = approx. 120lbs +/-)