How exactly do you figure seaweed or fish emulsion isn't organic? ;) And no, I wouldn't cut up your old sushi seaweed sheets (really biting my tongue on that one because it's seriously cracking me up, I find it pretty adorable). Whether or not your seedlings need more fertilizer than provided by the worm castings is really a pretty complex question, depending on what else is in the soil, what type of plants they are, how old they are, and how fast you want them to grow before you can plant out in your zone. Myself, I don't bother too much with extra ferts unless I need to stimulate growth or the plant is a heavy feeding variety, but when I do, I use a very diluted fish emulsion and only on mature seedlings (several sets of true leaves). It sounds to me like you've got the seedlings in some type of soil, ie not sterile seed starting mix that has no nutrients at all, so they're probably fine as is unless you have a reason to think they need more ferts.

Yeah, I figured seaweed and fish emulsion were organic, but honestly the rule for a lot of products seems to be they are chemical based, so that is always what I assume if I don't know. My other problem is that none of my local landscaping and gardening centers are open for the season yet, and they do not carry these products at Menard's or other area chain home improvement stores.

I planted a variety of flowers (cosmos, poppies, butterfly weed, dahlia bulbs) and herbs/veg (parsley, rosemary, oregano, leeks, tomato). Some plantings got the worm castings, but I ran out before I was done getting all the seeds in. I used regular potting soil to start everything.

Right now most things only have 1 set of 2 leaves. Sounds like I don't need to fertilize yet, but if I do at some point, I'm wondering what is best. Thanks.

The fish emulsion I use is actually certified by OMRI; there's a link at the bottom. So it's definitely organic. Since you started in soil and not sterile mix, and since your plants all seem to be doing well, and you're probably another month or so from planting out, I'd sit tight for now. You can always wait to fertilize until you plant out; bigger plants now just means more potting up and space issues w/ your lights anyway! :)

Here is a link that might be useful: fish emulsion