This is such a common misconception. 150w of MH lighting gives off the same amount of heat as 150w of T5 or a 150w light bulb or a 150w hair dryer or a 150w fan. A watt is a watt and always gives off the same amount of heat. The difference is in how the heat is dispersed. I run 525w of MH with no chiller and my tank temp is steady at 80-81.I really don't want to have to buy a chiller right now so I am looking into ways around that.