I've been using concentrated kelp. Seems to work well.

I try to add rabbit droppings or worm castings to everything that we grow. I have trouble finding ‘clean’ fertilizer from external sources, so we make this ‘In house’… Are you thinking of a product that you can buy and apply? Also, do you mean something that is easy to apply to seedlings whilst in seed trays?

Where are you located? This is for seedlings in trays? Any fertility in your seedling mix?

If you want something water soluble and simple, some kind of organic liquid fish hydrolysate fertilizer, ideally with humates and kelp as well. I get my transplants all the way to transplant stage without any added fert by adding compost and a number of other amendments to the mix, including a pelletized organic fertilizer available in our area called MicroLife. I also use their liquid fert called ocean harvest for a root dip before transplanting in the field, if you look that up you'll prob be able to find something comparable in your area. #grownostr