if your running DWC your roots should be in the water! if there not long enough (the roots), you need to use drip tubes to feed from above untill roots hit the rez, or add some serious airstones to create an aeroponic environment between the surface of the rez and the bottom of the pots.
i cant really tell if that a clone or seedling, looks like a rooted clone to me tho... if thats the case (clone) you need to be using 1/2 strength or stronger nutes... a clone is the same age as its mother, so it will need strong nutrients to grow, but @ low concentrations. you cant go wrong with a 20-20-20 nute at low dosages