I am pretty sure light is not entering the res. I am using the waterfarm res which is tan color. HOWEVER, yesterday I did the following:

Installed the Ice probe
Got a pink roll of insulation from HD; not the kind for the walls, its about 2 mm thin and can used in ridge vents. I wrap the insualtion around my res and 4 waterfarms. I then took blk/wht poly and warapped the res and 4 waterfarms. This will now prevent the lamp from heating up the thin plastic walls of the res & farms thereby heating up my water. Along with adding more reflection.

As of last night and this morning, my temps were down 4 degrees and the probe has not been on continously

The insulating part I feel is one of those steps everyone should consider doing if they have this system. It would seem as important as making this system recirculating and its easy