Seedlings tend not to need much in the way of fertilization (generally because by the time they would need a lot, they're ready to go outside). Any general-purpose formulation will do, following the dosage recommendations (or diluting those a bit) on the product label for plants in containers (or indoor plants). Liquid forms of fertilizer will be absorbed faster than granular (like the time-release types used in some potting mix brands), but the trade-off is that liquid types can also leach out of the pot faster the more often something is watered.

Seedlings are sensitive to soil moisture, not wanting to dry out too much, but also not wanting to stay very damp. When growing under lights, the light source should be turned on a minimum of 12 hours a day; 14-16 hours a day is more typical, but this can depend on the intensity of the light source. By giving plants long "days" and shorter nights this way, you give them the signal that it's the growing season, since some plant species that prefer spring or fall seasons for growth (like kale, for example), will respond differently in leaf growth versus flowering depending on the daylength they perceive.

The hardening-off process will help seedlings sync-up to current conditions outside and help support more growth. (More growth means more fertilizer can be used, to a point, making dosage and the frequency of application easier to figure out, since they will be cycling through a more "normal" level of nutrients as they mature.)

Miri

Thanks Miri!  

You're welcome!