trying to decrease the heat of my tank

You can use any fan as mentioned, including a cheap clip-on. They are most effective if they blow across the top of the water, however still work fine blowing directly on water as well. The back of my stand is completely open, so that is where I keep my fan. It is a PWM controlled computer case fan that is controlled by my Reef Angel. It comes on once the tank reaches a certain temp and then the speed increases if the temp keeps rising. Does a very good job of maintaining temp in that system (55 gallon w/ 30 sump).

 

84F is the temp found in the natural reefs

 

I'd consider a top (screen, eggcrate, etc.), as many folks are finding out that ANY fish can jump...we've had lionfish, and even bottom-dwelling scorps and stingfish that have gone floor surfing...fish you'd never think would jump in 1000 years.

Remember, natural reefs have a virtually limitless supply of water and oxygen, and the fish aren't so crowded together. There are also cooler places the fish can retreat to, where cool water upwellings or currents occur.

In a captive system at 75*F or more, the bacterial count goes way off the charts, the dissolved oxygen is lower in warmer water, and all of the water is the same temperature wherever the fish go in the tank.

Besides that, if your tank is running in the 80's, you have zero head room for temp spikes before things start dying.

There's really not a whole lot of good that can come from keeping a tank warmer than about 77*F, and there are certainly more reasons not to run a system that warm than there are to run it a bit cooler, IMHO/IME.