Try keeping the top open - if that doesn't help enough, get a large fan that you use in your room, and blow air above the water. This cools down the water quite fast... I only need to do this about once a year where I live, but it works!
im having the same probs as well i turned my heater off for the past few days..added cold water and left the lid open..but mines still shoots up to around 84c everyday!
What's wrong with 80? 2 above normal? The heaters not faulty. My old heater raised the tank to 100 a few weeks ago, and my severum, firemouths, and rainbow cichlid were fine. My Knife fish however Just turn the heater down a bit, and unless you have coldwater fish you should be ok
When my temps get a bit high in my tanks I like to use a 1/2 gallon milk jug that I fill 3/4 of the way and freeze. Drop it in the tank and it works out just fine.
I have had the same probs with temp because of the heat in general at this time of year. I was advised to put icecubes into a plastic bag and float the bag on top of tank water. It brought the temp down great and not to quickly.
Hope this helps