I would recommend using a water soluble, balanced fertilizer. Miracle-Gro or Peters for example, 10-10-10 or 30-30-30 or what ever as long as the numbers (NPK ratio) are about the same.

Dilute it 1/4 to 1/2 recommended strength and water the seedlings with it either every time or every other time. Depending on how much light they are getting and the temperature they are growing in. As long as their color looks good, they are not being over-fertilized.
Ed

Thanks Ed. I thought I kept a link to a post about this but somehow it disappeared or I've mislabeled it.
My seedlings that I started are growing fast and it won't be long before the 4 leaves show on most of them.

Julia

Ed
Will they turn yellowish if they are over fed?

dreaming,

Brighter than normal green leaves. Tips of leaves burned or deformed leaves would be indicators of over-feeding.

Turning yellow would indicate to me, a lack of something. Light, water, air in root zone, warm enough temperature, lack of fertilization or some specific nutrient.

We are talking seedlings in small pots planted inside.
Ed