current 1/10th < brand and size,
drew i dont use one on my sh tank but i keep it around 72 degree's, i have fans i put on there and with the leds really no heat
84 is a little on the high side. Try blowing a fan across the top of your sump. It sounds like you're going to need a chiller soon. In the meantime I would put some ice in plastic bags and float them in your sump to bring temp down. Fill some 2 liter soda bottles w/ water and freeze them for the future.
with Jim on the fans. Richie & Thane had me do that when I spiked to 85. you can use the calculator on BRS for the chiller, but 1/10 should be good enough... but for smaller tanks like ours...the fans work great..those tornado fans from target are cheap & do the job