lower leaves turning up with spots [pics]. Whats the problem?

One of my plants has one lower leaf curling up and has spots. The opposite leaf has spots. Would appreciate any feedback to fix this.

My setup. This happened in a home made pc grow box with 2 x 23 watts cfls of 6500k. Aluminum foil. I was testing this setup when it happened.

I have now moved back the plant to my larger grow area where it was was fine.

One reason I can think of is heat stress. The lights were much closer in the new setup... about 3 inch.

Another reason I feel could be infection. If so, should I keep this plant away from others?

Thanks in advance.


EDIT: (more details) Not having ph meter. Have ordered online. Where I stay, no one has heard of "sterlized soil" so I used a "potting mix" made by a local gardening shop. Light schedule is 24 on. temp 28 to 30 deg. Watering when top 1 inch feels dry... every 2 days.

with the info i just read and your pics, i would check the foil, maybe pull the lights away a bit, but not too far or the plant will reach. But, i dont know your ph, or nutrients so.............yeah

what......is that medium? let's start there