DIY chiller

[DiverDude]

Look for a water cooler -the kind that takes 5 gal jugs. What you have right now simply does not have the thermal capacity to deal with the volume of water you have to cool.

You could make a down and dirty chiller by running a few loops of tubing through the water-holding portion of the water cooler. However, you need to be very careful because you need to ensure that the chiller doesn't over cool the tank either. Now you're into measuring temperature and feedback loops.

Frankly, I'd suggest you implement some simple short-term fixes and shop for a used chiller. Point a powerhead at the surface to get is disturbed. Raise your lights if possible and/or look at shortening your photoperiod a bit.