A clip on fan or any fan that can push 60+ cfm will cool your water. My water normally gets to about 82 during the high point in the day. With a dual clip on fan my temperature ceiling is 78 degrees. I got mine on Ebay for 16.00. I opted for an unbranded Chinese version or you can pay 30% more and get a name brand like Zoomed or Aquatek. You can also spring for a Walmart clip on fan for 6.00, but those are fairly ugly, but it all depends on your needs.
Search for Cooling "Twin Fan (2 fans) Aquarium Chillers 80L" on Ebay if you want to see what I'm using.
I believe the temperature reduction is mostly due to the fan displacing the heat generated by my lights which would just sit there otherwise.