I don't see how an airpump could help. They get hot as they work and so you will be blowing warm air into the tank.
If you can't control the environment (air condition, move to colder area), your choices are pretty much
1) Take down the tank (or don't keep fish/plants)
2) Buy a chiller
3) Rig up some intricate evaporative cooling (fans blowing across the water surface, evaporated water added back via distilled water/top-off)
There have been a number of threads about hot tanks in NYC in the last few days, search for them as they might offer some better ideas.