Yellow lower leaves - 5 week old plants. Please help!

Leaves of my plants started to turn yellow, almost white on bottom branches. Plants are 5 weeks old grown from seed and currently vegetating. Is this a sign that my plants lack nitrogen? Should I start fertilizing them? I don't use any ventilating fan so could it be that humidity build up can cause this?
I water them every 3 days with pure water. Temperature is about 74 F and soil pH about 7.

Please tell me if you know what's wrong.

Time to give them some food. A 'grow' fert with micronutes.
Also, make sure you are using water with calcium in it; tap water is usually fine.
You NEED ventilation right away or you risk fungus and suffocation!
Other than that, you look like you will be in great shape as soon as you feed those lil suckers. They are nicely shaped, so your light is obviously adequate.