Seedlings do not need any feeding until after they develop their first true leaves, if even then. When I did feel the need for feeding my seedlings I used a very dilute fish emulsion, like 1 part fish emulsion to 9 parts water.

Get a liquid organic fertilizer that is pretty even across the NPK or maybe a little more on the P and less on the N. Then dillute it. But only after they are getting near planting. Quick to plant crops don't need any but crops that are started weeks ahead like a little dilluted fertilizer.

I am going to try some fish tank water now that I have a fish tank, and they sure poop!

Sometimes additional OM in the potted plants that are inside will be more susceptible to various diseases. Compost is the natural seed starter and will easily support the needs of young plants until they get outside. IMHO. :)