Hi Chiller experts,

Need help here. I had bought a second hand chiller Resun CL450 and i hooked up to my 2x 1 feet cube tank with a 1200L/hr filter canister.
I set my chiller to a temp of 25 degrees and after it had chilled to 25 degrees, the chiller light is turn off. But my chiller keep on chilling to reach below my set temp of 25 deg displaying 18 deg with the external heater light turns on. PLease advice on what had happened as my tank is has a water temp to 18 deg.