Those seedling look like they need some light.

You can transplant them after they have two sets of leaves to be safe, but you may be able to as early as one set of leaves.

If you wait till they have two sets,just clean off the coir and then bury the roots carefully in the hydroton (clay pebbles).

They will grow well in hydroton. Make sure you soak the hydroton in water and clean all the nasty crud off it before putting into the system.

If you don't get them more light, they'll be too leggy by the time they have two sets.
I always transplant after one set of leaves unless I'm just busy and don't get around to it until later. I say the earlier you transplant the more time the plant has to develop roots in its final resting place and thus the more time to grow without shock. the difference between one set and two sets can be anywhere from a few days to a couple of weeks.
What are your growing?
With DWC I personally would go with hydroton. Coir seems to hold a lot of water. you could also go with river rock or, if your pots are large, lava rock from the hardware store. Marbles will work too.

I agree that they are leggy. Are those tomatoes? If those are just cheap tomato seeds, since it's so early anyway, you could just pull them and start over. Or when you transplant them, just stuff them farther down into the next media. Everything you bury will grow roots. All you need is just the leaves above the media. People think tall is good, but most of it is just wasted space that makes it harder for the plant to support itself. Next time, keep your light as close to them as possible. A gentle breeze from a fan helps, too.

It are Lettuce seeds, it's true that they probably need more light, i only have natural light (sun) at the moment, but i want to buy some bulbs or a Fluorescent lamp. Which lamp would you advice me to buy? I'm also not sure if lettuce is good to grow for a beginner, and how to grow. I tried to transplant them to a simple system like this http://www.instructables.com/id/Compact-Cheap-and-Expandable-Hydroponics-System/ but my plants are just so small that they are crushed by those pebbles?! Anyone who can help me ? :)

don't worry, the hydroton will not crush you plants.
I looked at that instructable and you should steer clear of it. just check out the comments for a few of their problems.
for lettuce, just built a simple raft system using 1/2" styrofoam board and a square or rectangular container.
This is what I am talking about:

It will save you a lot of hassle. If you want more plants simply use larger containers or more of them. Those plants grew pretty well though it was really an experiment in using LED lighting.
A few notes:
Those lettuce plants are growing in 2" netpots filled with hydroton.
The black hose in the top right corner of the container is an air hose going to an air stone below the raft.
Paint the container black then white to block light from the reservoir to limit algae (some still grew around the edges of the raft)
ETA: if you don't want to use or have access to net pots, simply use some yogurt cups and cut or melt holes in the sides of them.

This post was edited by grizzman on Fri, Dec 28, 12 at 9:22

I like the extra square of styrofoam around the net pots. That is a good idea.

Thank you grizzman for your information! But just to clear some things for me up i have a few other questions, Do you germinate the seedling in a propagator, or do you put them immediately in the hydroton? And they'll need much light or not? I live in the Netherlands, and at this season we have almost no sun outside..Thanks in advance!

I germinate seeds in a damp paper towel(usually) then transplant them to the hydroton well before they have any true leaves. As for light; well I usually grow my stuff outside. If I were to grow indoors and was growing lettuce, I would probably use T8 flourescents about 6"(150mm) apart and maybe 2"(50mm)above the top of the plants.
@ Cole
I added the extra square because I felt the medium was too submerged without it.

It may not work for everyone, but I am having good luck with pine wood shavings as a growing medium, and Perlite is easy to use.

I have also had very good luck with washed gravel as well. I am currently using pine wood shavings for tomatoes with good results this year. I get compressed pine shavings bales from a livestock feed store.

With reference to searching the archives, there is not much of a chance that the original posters will be available for any clairification unless it is one of the few that seem to always be here. So, my recommendation is to try and get a current opinion and interchange going first while using the archives to augment what real live current responders have to offer.

I see many responces based only on what a responder has heard or read rather than the sharing of first hand accounts of a real successful application.

The lettuce seen here was started inside under lights, and then transplanted into a baked clay medium that is known under various names.

Thanks for the replies, but i still have problems..

The seeds i have germinated have almost no roots, i have germinated them in cocos coir, but it seems that it's not working. I have heard that rockwool would be perfect to grow them, but i can't get them here in holland :/....if i put such a small plant in the hydroton he dies after one day..anyone who can help me

Thank for staring threads and reply. Thank "timoson"for the question. I am also facing same problem.

Timoson;
Just keep then in the coir until they develop some roots then. Feed it with a weak solution until some true leaves start.
also, be sure to bottom feed the coir containers to encourage roots to grow downward.

For germinating seeds, you can just use water for the first 2 days. Once you see the seed starting to unfold, feed with a week solution and place them immediately under lights. Be careful not to scorch them.

I had a whole tray of seeds failing to germinate. Was wondering myself until i lessened the water. Too much moisture will rot your seeds.

When they have germinated, i find it better if i bottom feed them.