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How often are you watering? The top of the medium looks pretty dry. A good way to check when they need water is by lifting the "pots" up when they are dry vs just after you water them. You will be able to tell when they need watering by how heavy they feel. Might be under watering... Have you been checking your PH?
How old are they? They could probably use a transplant. The very bottom leaves (really yellow ones) are just the cotyledons and they are temporary. Looks like they could start using a very diluted mixture of nutrients but your new leaves on the growth tips look okay.
The medium should be good, never used it myself but it looks okay. The website says to start using nutes after the first week. I would check the ph of the run off water. The plants might also be getting root bound. I had to transplant mine a few days ago. A bigger pot and some nutrients will do them some good.