Lower the temperature. Once germinated, onions don't need 70F. they sould fine around 55-60. Yellowig can also mean lack on "N"itogen.

seysonn,

I've tried nitrogen with no luck. I'm applying it as a topdress fish meal. Maybe this isn't enough? Also, checked temperature again, it sits at about 67. Do you think this would be enough to dry out the and curl the seedlings? I appreciate your response.

Did you try trimming the tops off?
YOPPER

There are many more folks on GW who are more experienced than I am regarding starting onions from seed, so I do yield to their opinions...

My observation on GW has been that people tend to way over-fertilize seedlings in general. I use plain rain water (never city tap or well water) and I do not fertilize until the seedlings have been transplanted into larger pots. Then I fertilize with a half-strength solution every third or fourth watering until the plants go into the garden. When they have adjusted in the ground then I go to the full strength fertilizing.

I do not start onions from seed every year but when I do I start them in potting mix and it never occurred to me to fertilize the seedlings. Never had any problems.

dylcook, my point is that I wonder if fertilizing the seedlings burned them? Or maybe overwatering?

Hope you find a solution. :-)
-Tom