I'm by no means a lettuce expert as I haven't grown it, but from just looking at the pictures, they appear kind of spindly. This is normally an indication of not enough light.
How close to the plants do you set your fluorescents?
Is the fixture in the last picture your only light source?
If so, I would definitely say you need more light.

I like your set-up. Very professional, and I like your "admin"-writing on the alu-foil. Neat, but I think that I agree with grizzman otherwise.

Not enough light and they are getting leggy.

Takes about 30 watts/square foot.

Yes that the only light source for the plant. The light is about 1-2 feet. Maybe I will add another one.

yes and keep them closer to the plants.
for your setup, you could use 18" fluorescent tubes, either 2 or 4, and keep them 2"-3" off the plants.

Also, consider buying a fluorescent light designed for plant growth. Standard 23W bulbs look fine to people, but plants can't use a good bit of the light they emit.

Are those light used for aquarium plant suitable for hydroponic?