Goldfish can do fine into the 80's, as long as there is adequate oxygenation. My tank routinely gets that hot in summer (as do ponds around here), and the goldies are just fine.
For the best aeration, some kind of a water-fall or wet-dry filter is best. <Large surface area for gas exchange with all the droplets, and the evaporation also cools the water a bit.> A water fall is not too practical for small tanks. Instead, you can arrange to have your filter splash the water around. <Drop the water level, or raise the filter outlet.> You can also add an airstone or 2, but splashing, although messy, is prob. more efficient.
If you are getting direct sun that is warming the tank, a curtain will do wonders. If the lights are contributing to the heat, then venting & fans are in order. You can also blow a fan across the top of the tank to cool it down by a few degrees. <With either splashing or fans, you will have increased evaporation, so will need to top up the tank frequently.>
You can get an aquarium chiller to control the temp, but that is not really necessary. It is expensive, and for the price, you can just get a room A/C and cool the whole room down.
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