[Help] Week 4 seedlings show yellow/brown spots on leaves

Hello, could you please help me diagnose any issues with my young cannabis plants? I'm starting to notice light yellow/brown spots on a few leaves. The spots are found in the interior of the leaves, while the leaf edges are OK. The leaves seem to crack in the spots when bent.
  • Age: week 4 (from seed)
  • Environment: outdoors, 8+ sunlight every day (dry, sunny California)
  • Growing medium: Fox Farm Ocean Forest
  • Fertilizer: none added so far
  • Water schedule: every 3-4 days, regular tap water
  • Other info: Transplanted these babies a week ago from solo cups to 2-gal pots
 

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