call me crazy but I still think 81 is ok, if you hit 83/84-ish then you should be a little worried. leaving the hood open with a fan blowing on the tank and water might help... the tanks water will get warmer if the air temperature around it is warm too, do you live in an apt?
you can get a plastic ziploc bag and put some ice cubes in it and float that in your tank, very temporary though... (make sure no leaks first)