really any type of new soil that you use (or soiless mix) has natural nutrients in it, which your plant will use during the vital first stages of your grow. Give them time to absorb what your mix has to offer, and then you can add a light dose of nutes. this is why everyone says wait until week 3 no matter what to start ferting your plants. that, and your plants probably wouldnt be able to take any type of additional nutrients in its seedling stage because theyre so small. i actually tried to test this theory out with this grow that im on right now, and i ended up burning my plants a little bit. My soil is made up of FF ocean forest, Perlite, Dolemite Lime, and a tiny ass bit of vermiculite.
The best thing you can do right now is to just use smaller starter pots for your seeds, to get them started, because its easier for your lighting system to heat up a smaller pot, and keep the soil completely warm, as opposed to using a huge pot, and having that little seed in the center get cold, and die off on you. so use your soil mix (premixed before you put into the starter pots), and start the seeds off in the little pots for the first 2 weeks, and then transplant into your larger pots for the rest of the grow, watering them in with just water. once the plants have adjusted, and have about 4 nodes, then hit them with a light dose of nutes on the third week - about 1/4 strength the recommended doseage. After that, just slowly start increasing the doseage until you find out whats right for your plants. just remember, give your plants nutes with every other watering once you start - you dont wanna burn out your plants, plus this will also slow down any type of significant nutrient salt buildup, by giving the plants ph'd water every other watering.
So at three weeks - 1/4 strength nutes in your water. when the soil is dry again, feed with just ph'd water. next time soil is dry, nutes again, and so forth and so on.
hope that helped a little bit, and i also hope im not repeating what other people on here have been saying to you the whole time, lol.
-K1