I have a couple plants that are about 5 weeks old, and one has started getting yellow edges around the leaves. I feed them with worm tea, kelp, and calmag every other week, but it's a light dose so I think this is a deficiency and not a toxicity.
Next time, just water them and watch. Set your light correctly (40cm/PPFD) according to the plan and see. You don't need to worry too much about that; it just shouldn't get worse.