Thanks, stinkin...and that's what I thought but I've tried everything else! I've cut back on the feeding and that didn't seem to help...I'm diligent about checking the pH of the Distilled Water I'm using...I don't let the girls sit in water, I pour it off...perhaps I should cut back to 400w instead of 600w?

Interesting. I also use a 2x4x6 Hydrohut and am right at week 4 in flowering. I am getting some yellowing/browning of the oldest leaves on my plants, as well. Sometimes I think that my hut is too small for the 600w hps light that I use, although the temps at canopy height are not too high (highest it ever gets is 85 and that is rare), but it justs seems like the light is maybe too intense for the plants. I know 75w per square foot is supposed to be ideal, and 75x8=600 (the sq footage of the HH base is 8 ft), so it seems like 600 should be perfect...but sometimes I think it may just be too much for an HPS light in that size hut without a lot of area to dissipate the heat and circulate air, even with a good exhaust fan, which I have (Vortex 6")...I definitely notice the plants directly under my light are a little "bleached" compared to those on the perimeter. Keep in mind, I have no air conditioning...so whatever the air temp outside is, that is what I have to work with. I have a feeling that if I could regulate my air temp at a nice and cool level during the light cycle, then everything would be okay. It is pretty much always 70-75 here during the day and cool at night, but the 70-75 day temps are a little high for fresh air intake. Overall, things seem to work out well. But I do agree with you that the 2x4 hut may be small for a 600w. I would be REALLY interested to see the results of a 400w in one, in terms of heat/humidity/yield...