[hahnmeister]

Well, what sort of air movement were you using with each one?

According to calorimeter tests, the T5 and halide technologies make about the same amount of heat (T5s actually make slightly more, which makes sense, since their light output is slighty less at the bulb). If the halides had the as much fans as a T5 setup, they should cool just as well, if not better (more open water area). There are too many variables here to argue... something as simple as what skimmer you were using then vs. now, or a change in circulation pumps could also account for the change. Either way, we are arguing about pretty small variances here. The most efficient halides still are converting about 75% of their electricity into heat. The most efficient T5s are converting say... 80% into heat... so we are talking about ALOT and slightly less of ALOT. It really comes down to the fixture and the ventilation method of the tank more than the lighting technology itself. We are comparing 80% of 600 watts to 75% of 650... whoopie... either way, you are still putting a 480-488 watt heater over your tank. Either one could heat your tank up well over 100 degrees if not dealt with properly... so how you deal with it will mean more than if its T5s or Halides.

From my dual 40B setups, where one was lit with a 250wattDE and one with a 6x39wattT5, the T5 tank heated up much more when the lights were on compared to the halide. Both were open top, and the halide left more open space for cooling while the Tek made a pocket of hot air across the entire surface of the other tank. The halide tank was a happy 80 without a fan, while at the same time I had to add in fans to cool the T5 tank or it sat at 85 degrees.