Unless your seedlings are more than 8-10 weeks old and still in a soilless growing medium, they probably don't need to be fed. Most growing mediums that you would pot up with contain sufficient nutrients to carry the plants to the transplant into their final home stage.

Betsy

I would start fertilizing at 4-6 weeks or when the cotyledons dry up and fall off. They are what holds enough nutrients to get them through to transplant stage IMO but if you used a potting mix with fertilizer instead of seed starting mix then they should be good to go until planting out. Fish emulsion is normally mixed (depending on brand) @ 2 tbsp per gallon. The conversion for that would be 1-1/2 tsp per gallon for 1/4 strength. Personally I would use something higher in phosphorus for good root development instead of nitrogen for lush foliage. Top growth matters very little until after you get them in the ground or permanent home but with good roots comes good plants that can pretty much support themselves in good soil.

My plants are still in seed starting mix, although they have been replanted into 750 ml yogurt containers.

They were planted on March 23, and got true leaves between April 5-10th.

They seem to be doing relatively well since it's my first try, although they're looking a little weary. Not droopy, but just not thriving.

When I repotted them they had a pretty good root system. I base that on what I've seen on plants I've bought previously.

So, fish emulsion at 1/4 strength until I get out to buy something with more phosphorus?

Thanks for your advice =:)

Since you have the fish emulsion, go ahead and use it at the 1/4 strength. If you know what the brand is, you can Google it and find out what the recommended mixture rate is. If you can't even read that much on the label, I'd suggest erring on the cautious side and maybe mix one teaspoon or less to the gallon. You don't want to bring them this far and then fry them by over feeding.

When is your plant out date? If it is more than a couple of weeks away (which I suspect with a zone of 2b/3a) you may want to seriously consider potting them up with a quality growing medium that has nutrients that the seed starting mix does not.

Good luck!

Betsy

Our plant out date years ago would have been any time after this long weekend, but generally accepted practice is more like June 1st now.

Well, I took your advice and watered about 8 of the smallest tomatoes, plus a few of my smallest marigolds and I'll see what happens.

If I did them harm, how long will it take before it's evident?

Not all fish emulsions or fertilizers are the same. Check out manufacturer's instructions. For the past five years or so, I have been using Neptune's Harvest Fish and seaweed fertilizer at 1/2 tablespoon per gallon for all of my indoor started plants after they have been transplanted to larger pots prior to putting out into the garden. Have had no yellowing problems during my hardening off of the veggies. Plants are big and green and vigorous. I use Pro-Mix as a potting up medium. No special MG type additives... just good starting up stuff. Hope this helps.
Victor