I'm having the same problem myself at the moment.
Over-watering can cause these symptoms, but in my case I know that wasn't true. So I checked my water ph and found it to be a bit high. I got my run offs down to 6.8 and am waiting patiently to see if they improve.
RAINHAZE Reviewed by RAINHAZE on . need help with seedling problem I have grown outdoors for 38 seasons this is my first indoor- growing medium is 4 cubic feet pro-mix ph balanced at 6.5- added 15 pounds worm castings- with 1/2 bag of Happy Frog and 20% perlite- this mixture highly recommended by my equipment supplier- growing under a 1k MH at this time, temps are 76-to 80- 24 inches from canopy and 68 to 65 for night temp- plenty of circulation- RH between 57 and 20 - drops pretty low when heater comes on, added a humidifier this morning- Problem is seedling Rating: 5