There are several different methods available to you for this problem, all of which work well, we can attest to that. For the environment around the tank and the entire house for that matter, possibly, an air conditioner could prove most helpful. Cool air surrounding the tank will help assure that the tank will stay cool also.
Along with the lids beng open, try a fan across the top of the tank, can help to "blow off steam" as it were. Air circulation is also good for the tank itself.
Finally, if it's an emergency type thing and you need rapid cooling, try Ziploc bags, or any other bag that can be sealed in some manor, filled w/ ice. This won't harm your fish as the cooling is gradual throughout the tank as a result of the circulation of the water.
In addition, you may want to check your heater, if there is one present. If it malfunctions you may not know or you may blame it on the heat. Albeit rare, it does happen from time to time and may be worth checking into.
Hope that did some good.