ok im just about ready to give my first grow a try and ive read all sorts of things on here but i still have one questions concerning the seedling in a DWC.
once its germed and has taproot showing i then place it into my medium(mine being rockwool).Now with soil i know ur not supposed to add nutes till the second set if true leaves come or three weeks(a little discrepancy there) but with soiless, i can only assume u add nutes almost right away(my guess, once the cotyldons have formed) So when do i start to add nutes, and whats the amount(like a 1/4 strength?)

Well dude, im 3 weeks into my first "successful" grow, i have lost lots of seedling already but now i have 4 3week old little plants growing in rockwool with a gravel ballast surrounding and covering the top and bottom of my cubes...

Problems i found and had plant fatalaties!!! 1 didn't soak or rinse my rockwool with the correctly pH adjusted water....... they died .. ahhhh

2. added nutrients too early and this burnt the roots!!!!!!! they died.

3. used mineral water which might have contained too much dissolved salts so i changed to tap water and i think their health improved.

so eventually to have a success i prepared my grow medium "rockwool and gravel" in "distilled water pH adjusted to 5.5 would be ideal" but tap water left to sit overnight and then calibrated to pH 5.5 "i used lemon juice for pH down and baking soda to counteract if i needed to bring it back up" has worked and is what i used.... i watered once every 2 to 3 days just as the lights were being turned off and misted daily once at the same time again once they had their first set of true leaves ie. "after the cotyledons".

Then the nutrient question, heard lots of advice and suggestions and what i done yesterday is i prepared a solution 50% strength of the recommended quantity stated on the website and on the bottles ... i then fed my 4 plants 1. with just water, 2. with a 25% solution of the 50% solution and so on if you understand so i have 4 plants all recieving different strengths in a 4:0, 3:1, 2:2, 1:3, water/50% nutrient solution ratio... so i'll see what happens and which plant responds best "trial and error" i began feeding as the plant leaves were yellowing but there was always continous growth so i just decided to keep one only feeding it water... "i was told about the node time frame also" technically the plants are on their 3rd node but they are very small..

i am using a 250 watt metal halide light on an 18/6 schedule and i have a 40 watt household fan blowing at minimum speed every 45 minutes during the day cycle... the peak air temperature in my room hits about 27 degrees celsius at the plant level...

so i hope this advice helps ...mine are growing "fairly well" and im happy with the progress and also it starts to smell now which is a great victory!!!!

So to finish "use water to clean the rock wool and supporting medium" pH 5.5.

Water with 50 to 100 mls of water every 2 to 3 days use you own judgement as with mine i left trays under the pots so maybe if they are suspended they might need more frequent watering ..... visually inspecting the top surface of the rockwool should be a good guide.

and then if you observe continous growth you can make a plan on what to do next... my plants/seedlings lower leaves went yellow and brown very early as i was only feeding them water "but "new" growth was always observed" ..... so i began feeding yesterday as this is a sure sign of a nitrogen deficiency... so hopefully now i can observe which plant responds best to the new nutrient feedings and apply that ratio solution to all 4.....

Thats my experience so far so i hope it might help and good luck and happy growing!!!!!!!!!!!!!!!