I'm assuming your lights are on...

You've got a few options... I'll give them to you in order of cost lowest to highest...

Fans:

This is the most popular (and cheapest) way of keeping your water temp down due to heat produce by your lights. Basically, you need 2-4 4" fans (usually computer fans are used (80mm) in conjunction with a power adapter)... this can be accomplished with about $20.00 USD...

Better circulaton: The more circluation between your display and your sump the better when it comes to temperature. I say this because if you're display lights are normal, meaning alot better than you sump lights, they're probably going to produce more heat. This means that your water is being heated up more in the display than in the sump. SO, the more time the water is circulated in and out of the sump, the better. This is probably the least effective method of the three, and is probably not the cheapest.

Chiller: Pending on your tank size, I wouldn't consider this a viable option. Chillers are usually VERY expensive ($250 - $500+), and they're usualy used for larger aquariums (125+) and are most popular when MH lighting is used (due to the high heat output of MH lights). This method is probably the most efficent and effective, but its cost is its major deterrent.


Hope this helps!

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ALSO! Do you have any pix of your tank?! Seeing it could help me/us give you some more sugguestions!