i'm on a well and i have so much iron in my water if you drop a glass of it on your foot it'll break your toes.
but my water softener does a good job of making what looks like swamp water into pretty clear and potable water. maybe iron in the water isn't absorbed or maybe it is, i don't know. i have corno di toro, fatalii, long red slim cayenne, tabasco and datil peppers and the only ones that had a yellowing issue and were much smaller than the others (of those started 2/29 - remember the fatalii's were started in late sept) was the datil peppers. based upon marktutt's comments in a thread i started i now think it was too much water and exposure to temperatures that were too cool outside and in the basement under lights and especially when wet from watering. the fatalii were started in late september when it was warm inside the house and over wintered and did fine. fatalii and datil are the only peppers that are chinenese and they are more difficult and less tolerant of too much water and cold. once i let the datil dry out, never watered them unless they'd be in a warm spot, was light on the watering and kept them upstairs at night on the dvr at 82, once i did those things 2 of the 3 have recovered and look fine but one is still quite small tho much greener.
you don't mention varieties, watering, temps, are they severely root bound, how you are caring for them so it is pretty hard to know.
tom