You can add a fan... you can also add "bags" of ice You would probably want them sealed so you don't add a bunch of water to the tank. You could get a chiller. You could change your lighting so it doesn't add as much heat to the tank... but if you are having problems already... I think a chiller is your best bet (expensive and can get stuck in the "on" position). But a fan can do amazing things (does increase your water evaporation a lot.)
Do you think you could post some photos of your setup (showing air flow, lighting, and sump ect.)