Do I need a chiller?

[George]

Go to walmart or Superstore or Zellers and buy a couple of clip on fans. They are only $15 a piece. Try that before you invest in an AC. Plug them into your timer so that the fans are on when your main lights are on, if you don't have a controller. Blow the fans across the water surface.
Depends on what kind of light you have. If you have T5's, I am sure two clip on's will do. If you have MH, maybe sufficient, maybe not. Try the fans first anyway.
It will evaporate a lot of water if you do the fan route. JFYI.