The older leaves are showing a lack of water. That could mean only the roots by the newest growth are viable and providing water to the plant. It could mean the heat of the Halide lamp drew water from the plant faster than the roots could take up fresh water. It could mean there was not enough humidity in the area around the plant. First thing to check is the roots. Unpot, give the roots a 10 minute soak in tepid water, check to see if the older roots are mushy feeling and remove any that are rotting and repot. It should be able to handle the higher light output of the halide lamp, and the higher temps near it. The second step to try would be a humidity tray beneath the pot. I have been using plastic see-through drip trays with a double layer of marbles filling the tray. Set the pot atop the marbles and fill the drip tray to just below the top of the marbles with water. Keep an eye on the water level, it disappears rapidly!