So what happens to plants under HPS light without proper ventilation?

I don't have anything to move the heat from the light.It's 400 wts. I didn't understand the importance of this when i bought my set up. I got my plants 2 weeks now under 12/12. I've had to move the light almost three feet above them because they were drying out too fast. As a result right now they have lost their leaves from mid point down. Does this sound like an air ventilation problem? From mid point to the tops they are fine. Can I get the things i need from Lowes rather than the hydro store?

you using a tent, or an open room?

I get by with 4" CAN fans, don't know if Lowes carries them.

Open attic. I enclosed a 12x6 area last winter with heavy plastic and reflective insulation on top of the plastic. Opened it up a little, enough to pull some cooler air in with a reg fan. What are the 4" CAN fans and where'd you get them?