As I said I chased that ghost (heat) before in a NFT system rez. Had myself convinced it must be the submersible pump, even tho it was on a 1min on/4 min off cycle. Went to the hassle and money of getting an external pump only to find it made no difference in temp. Since temp ran slightly higher then room ambient I started looking at the air pump. Decided to experiment and placed it in a cooler with dry ice along with abt 10" of air line, soon rez was lower then ambient. I knew I found the problem then. Figured it was worth sharing.