Which controller are you using? I have heard good and bad things about controllers
http://www.etcsupply.com/ranco-etc111000000-prewired-digital-temperature-controller-p-87.html
This what I got. You can plug in up to 1500 watts of heaters if needed. Put the temp probe in the tank and put the heaters in the sump.
I have 4 of these and two Aqua Logic TR115SN which are exactly the same thing but cost more. I cooked a Cuban gar with a $10 heater and promised myself that will never happen again. Even my "grow out tanks" use these controllers now. These controllers add a layer of safety and they are very accurate. I have one on my Goonch tank just to monitor temperatures, no heater attached.
