Buy a fan to blow across the water.
That is a good way of getting rid of heat, but you will also greatly increase the amount of evaporation. With only 16 gals. of water to start with(or less depending on the amount of LR
and sand) that may be a problem. If you do use a fan, be sure to monitor the evaporation rate and salinity changes closely. You may have to do multiple top-offs daily. Another good idea is to bump your temperature up a bit to limit the amount of fluctuation during the day/night. FWIW
84 is not too bad, many tanks w/ MH
lighting run "warm"- although 80-82 is ideal for a reef IMO
. It's when you hit 90 things can go down hill very fast.