Lettuce on floating raft

I didn't write for a while because of massive amount of work and health problems. Therefore this time I'll briefly describe my experiment of growing lettuce hydroponically in floating raft system, also its results and conclusions.
3 days after planting lettuce seeds germinated. At this point I realized my first mistake. Every "How-to germinate lettuce seeds" manual I've read advised to water turf first and plant seeds afterwards. In a hurry I did it in wrong sequence - at first planted seeds and watered them afterwards. As a result in few cells I washed seeds too deep and they didn't germinate, in few other cells they moved and germinated too close to a wall of a cell.

I took a cover off my mini-greenhouse and left my lettuce sprouts to grow on a window sill under sunlight. They grew well. This is how they looked after a week:

In about two weeks lettuce sprouts grew big enough to be replanted into my hydroponic setup.

I put my floating raft hydroponic system near a window, turned its aerator on and filled tank with solution. I used Vito 1 liquid fertilizer dissolved in water in recommended proportions for this experiment. I had to adjust acidity of solution by adding a bit of vinegar to lower pH level above 6.

After filling approximately 20 l of solution into a tank I put a foam rubber raft on a surface.

Then I started replanting lettuce sprouts to clay. I took them very carefully out of turf and washed roots under running water.

Holding lettuce sprout above a pot so that its root hangs inside I carefully filled clay.

At this step I realized my second mistake. Clay granules were too big for such a little sprout. It was very hard to make lettuce sprout maintain vertical position. Next time I will use smaller granules.
As I finished replanting pots with lettuce sprouts were installed into floating raft hydroponic system.

Next weeks brought me problems. First of all - direct sunlight reached my sprouts through a window just till noon. Another half of a day they received reflected light only. Second problem started day after I replanted sprouts into clay. Sky became very cloudy and long raining period started. The sun was very rare guest whole next month and sprouts desperately lacked sunlight. In a week they grew just a tiny bit.

Whole next month was cloudy and rainy. This is how my lettuce looked like by the end of it:

One plant died and was thrown out. Others were very weak. Leafs were very pale, thin and brittle. Instead of growing upwards they just lied on raft.
I concluded that my experiment failed. It became obvious for me that I never should rely on natural factors while growing in artificial circumstances. Next experiments are pointless without artificial light. I ordered led-lamp with 36 blue and 90 red diodes and 15W overall power. Next experiment will take place after I receive my order. I believe that light will bring me success.