I don't know. I do know i have used heaters on 10 gallon tanks that were 200w and 300W and never had it overheat so size is not the issue. I also know that others have had 2, 100 watt heaters almost boil a 55 gallon tank, brand new out of the box. If it has a warrenty or the store will replace it, get it replaced.
Otherwise start freezing distilled or dechlorinated water and start adding it every hour.
kateyoup has a good idea and those heaters will work fine. Although the size of the heater is not the cause most of the time. I would still try to get the store you got it from to exchange it because it is likely faulty.
Please don't think that new things can't be bad, even heaters. I have seen and heard of a lot of brand new "TOP OF THE LINE" merchandice that was worthless "right out of the box." Heaters, pumps, filters, light fixtures, tanks, batteries, alternators, stoves, stereos, cars, trucks, and so on and so forth. Jusy because you pay $100,000 for a Mercedes doesn't mean it can't be a lemon and just because you pay $10,000 for a KIA doesn't mean it WILL be a lemon. Same goes for aquarium equipment. Just because it is top of the line doesn't mean that it can't be faulty and just because it was under $10 deosn't mean it won't last for 10 years.
A heater that is too big is NOT recommended because of one reason, if it does go bad, it will heat the tank fast enough and hot enough to kill all the fish before you notice it and in a very short time. A smaller heater may not heat it that fast and you could at least have a chance to notice it before it cooks your fish.
I don't mean to be critical but your fish will be one that suffer because of the heater. If it is that new, exchange it for a new one and if that one does it, get a different brand.
After all that, I do have one more question that could be the cause. Is the side of the tank that the heater is on against an exterior wall? If it is then that could be the cause of it heating up so hot because the heater thinks it is colder than your tank actually is. If not, get a new one through exchange if you can or get a new one, test it and if it does it again, then get a different brand.
