Tiny brown spots appearing on leaves

I know there's other threads out there about brown spots on leaves, but none of the photos of those plants look like mine, and nobody seems to have a definitive or conclusive answer as to what is causing the spots.

I don't have a decent camera that works right now, so I can't provide a good photo of the spots, but they're basically tiny little brown speckles or splotches that are appearing randomly on otherwise healthy-looking green leaves. The leaves don't appear to be dry or curling or whithering. The stems also look nice and healthy, although there is a slight purple tinge to the stems.

I'm only about 12 days into the grow, so I've been going light on the nutes. Started from day one with 1 AG starter/seedling nute packet in distilled water with pH steady between 5.5 and 5.9, and the 2 PPP seedlings looked nice and healthy. Drained res after week 1 and replenished with quarter-strength Fox Farm Grow Big and Big Bloom, premixed into 1 gallon of distilled water with a post-mix corrected pH of 5.8, and a PPM of only 160. Been checking pH and PPM at least twice daily, and pH and PPM have dropped gradually this week down to 5.5 and 145 this morning. Added 2 drops of pH Up to the res this morning before I left for work.

Not sure if I should add more nutes--thinking maybe the brown spots are from undernourishment, but not sure, since the plants look otherwise healthy and the brown spots so far have only appeared on one plant. That plant is closest to my home-made CO2 fermenter jug and the fan is about 2 feet away blowing gently on the plant-- just enough to make the plant jiggle slightly.

The res temps have stayed in the 75 to 85 degree range, staying pretty consistently around 80 degrees lately. I have a 24 inch flexible airstone curled around the bottom of the res, providing even and continuous water circulation and oxygenation in conjunction with the built-in AG pump.

Any ideas on the most likely culprit for the brown spots? They just appeared in the past 2 days on the one plant.

I would've guessed pH fluctuations, but that doesn't seem to be an issue.

Yeah, I've definitely been keeping tight control on the pH. I'm really beginning to think maybe I've been a little too light on the nutes. It was down to 135 PPM this afternoon when I checked it, so I topped-off the res with some slightly more nute-enriched water to get the PPM up to around 250 PPM. That may still be too weak, but I definitely don't want to overdo it from everything I've read here. I figure baby steps is better. Depending on how my babies fare the next couple days at the higher 250 PPM, I might try upping the nutes to around 400 PPM when I drain and refill the the res on Sunday. I figure if I'm up to around 800 PPM or so by the time I switch to flowering in a couple more weeks, that's a pretty good, steady ramp-up.

Dude it really could be anything. I know you said you dont have a good camera but without pics we are all just shooting in the dark. Hell, even with pics its still somewhat of a guessing game.

Starhawk: I wouldn't stress over it if the succeeding leaves are green and healthy. That seems to be a normal condition and what happens to mine at about the same stage when the seedlings start to veg. We get to be like nervous expectant fathers and we just need to relax and enjoy the process. If it was nute burn it would be at the tips of the leaves and it would be true for the new succeeding leaves as well. If they out grow it I wouldn't worry and in a few weeks you can just pick them off.