Seedling not growing/Leaf Curl down..

I have already read the sticky threads but still confused..

Medium: Coco coir& Perlite.
Light: Flourescent 25W

My seedling has been a week after planting in a plastic cup (4holes for aeration).

It was growing good for few days..
but is not growing after the first 2 leaves,
and and leafs are all curled down. color is still very green.
It just stays the same for 3~4day..

I thought it was overwatering problem..
So I left the surface dry for 2days but it does not seem to recover..
I watered them today..

Light is 3" away, Ph is 6.7~6.8 (No nutes), RH 45~50%, Temp is 80~81 F
2 Small PC (fan 120mm) is running in a tent for air circulation.

I really cannot find what the problem is...

Please give any advice for this newbie..T^T

It might be that water ph...coco water should be ph of about 5.8-6.2 could be nute lockout