Answer: You can begin thinning now, providing about a finger's width of space between each seedling. Clip or cut the stems of the seedlings you don't want to keep, or dig them out carefully and transplant to another container. Clipping will disturb the roots of nearby plants as little as possible, but digging will provide you with additional seedlings so the method is up to you.
Seedlings won't need to be fed until after they've developed their first set of true leaves. I use a diluted (one-quarter strength) liquid fertilizer to begin with and gradually increase to regular strength as the plants grow.
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