I'm getting the impression the tank is covered? If so, just removing the covers will make a huge difference. Fish jumpers can be contained with screen covers. Then you'll be able to run a small fan across the top of the tank.
Another option may be a small drop-in chiller that shouldn't break the bank. In any event, 3 weeks of evaporation will be hard to provide for and measure.
But I gotta tell ya, 3 weeks without AC on the 3rd floor in NYC, I think you'll be pushing your luck even if you have somebody looking in on the tank.