Doesn't look like heat stress, but they do look bleached.
1) When did this condition start, and are you using anything different than on previous successful runs?
2) Is this the same strain you've grown before, or is it a new strain to you?)
3) Is the tapwater from the city, or a well on your property?
4) What's it's out-of-the-tap ph?
5) What are you using to adjust water ph? (you're not adding limestone to the soil or water are you?)
6) What are you using to check runoff ph? Are you attempting to chase an invalid ph number with color-coded strips or kits?
7) Are you spraying with ammendments or 'growth' teas or bug sprays?
8) Doing anything to them that might affect roots? Stuff like consistent overwatering causing root stress/root rot, sitting on a cold cement floor or next to a heater, overdosing with hydrogen peroxide...)
9) I've never been a big fan of a 24/0 light schedule. Have you always used this schedule?

Most (peat-based) soils without ph buffers take a ph dive and go more acidic. Overbuffering will keep it near 7.0 no matter what you do. So doubtful it's the FF soil, but anything is possible. Is it fresh?
When do you plan on starting the FF Grow Big and Big Bloom? I know 21 days is a tad early for 21 day old seedlings, but did you 'accidentally' fertilize them too early? Ocean Forest is chock-full of nutrients already, but could be time to start half-doses per FF Feeding Schedule if you've overwatered/flushed too often. The link is in my signature. (if you try...only try on one till results appear)
Rusty Trichome Reviewed by Rusty Trichome on . Yellowing - Form Completed & Pics I don't know what to do. Soil runoff is at 7.5 even after flushing with 3 gallons of 6.8. E-indoor or outdoor - INDOOR E-soil, soilless, coco, aero, or hydroponic - SOIL E-specific medium - FOX FARM OCEAN FOREST - STRAIGHT FROM BAG CSL-Soil type/brand - FOX FARM OCEAN FOREST - STRAIGHT FROM BAG SCL-Anything you have added to the soil - PH'D TAP WATER SCLR-Soil or slab runoff pH - 7.5 E-Water source - TAP E-Source water pH - ADJUSTED TO 6.8 Rating: 5