Soluble Miracle-Gro or "Peters Professional" water soluble fertilizers.
But use it very weak! Like 1/4 strength, infrequently. Classic words of wisdom apply:
"weakly, weekly".
Even if you started seeds in something like pure vermiculite or peat or bark with zero nutrients, young seedlings are hurt more than helped by fertilizer unless they are old enough to be ready for it.
What do others think is a good time to start light fertilization? Two pairs of true leaves? Four pairs?
Maybe someone knows of a no-smell, fast-release organic fertilizer.