i've got this exact same problem in a soil grow. some threads covering the same problem suggest its overwatering, so i waited untill the soil had dried out (5 days in my case) and flushed the soil with 2 x the pot volume of clean water.

any crap in the soil has hopefully washed out, and i'm giving it longer between watering.

and hey presto! my babies DO look better:thumbsup:

i realise you're not useing soil, but i overcame it, so hopefully you can too.