When do you start fertilizing seedlings

I have found that 1/4 strength fish fertilizer does wonders for seedlings. When do you start to fertilize your seedlings? After the 2nd set of leaves form or before?

After first set of true leaves for me.

How do you apply the fish fertilizer?

I dilute it in a small watering can. The plant absorbs the nutrients thru both foliage & roots, so I top water when fertilizing, about once a week. It stinks, so you don't want the fertilizer to sit in the water pan under your seed tray.

First it depends on what your seedlings are growing in. Some potting mixes contain plant food that are good for 6 weeks, others don't

When I need to feed seedlings, I use Miracle-Grow diluted to 1/4 th strenght because it contains all the nutrients plants need.

You have to be careful so tender roots don't "burn" from over-feeding. I don't begin feeding until after their first set of true leaves are fully developed. And very lightly.

Let the color and growth of the seedling let you know if it is needed.

Thank you for the responses.

I'm growing tomatoes under lights for the first time and am about at the stage I want to fertilize as the mix contains no fertilizer. I read one post where the writer said he sprayed it on. I have the fish fertilizer but was afraid if I watered from the top that it might contribute to damping off. I was advised to water from the bottom which I understand.