Reef Tank Temperature Fluctuation

Hi everyone, I'm just wondering what your tricks or tips are for keeping the temperature stable. Currently I have my two heaters in the sump of my 125gal tank so it isn't an eye sore. However the temperature fluctuates from 76.6 all the way up to 79 during the heat of the day while it's hot out. I do have a lid on the tank to keep my fish in as I know that can trap heat but I'm hoping to keep my temperature at 77 degrees f.