Nobody answer yet? I'd like to hear what experts say about it too.

I don't know why your seedling turned yellow in the first place but fertilizing it doesn't sound like a possible solution. Normally I wouldn't use any fertilizer until the plant is much bigger. Even when I started to fertilize, I wouldn't use full strength either. Felt safer using 1/4 strength. Probably a little too weak but my rationale is: under-fertilizing is easily corrected, over-fertilizing is not.
It seems you use specialized fertilizer with extra P in it. I know phosphate help establishing roots and promote flowering but I am not sure if it's really necessary. If you routinely use it and never had problem with other plants it's probably alright.

Another thing I noticed is the seedling is in a peat moss pellet. I used to use them but found they gave me more troubles than their worth. When it looked dry on the top the bottom was still too wet. And water just ran through once it's really dry. So it's still possible the seedling was over watered even the fan was running all the time.

These just a few thoughts crossed my mind. I only have a few years of gardening experience. Hopefully it stirs up some activity to get attention from experts.

Thanks BloomCin I will check bottom of petunias to make sure they r not real wet when top is dry like you say. I will hold of fertilizer too for a bit. Thanks again