Tomato seedlings turn yellow - pics
What causes that? The seedlings are inside a very humid, closed propagator, right under LEDs running 18hrs / day. They're on a small heating pad. The rockwool cubes they're growing in were soaked in a pH 5.5 solution of fertilizer, dilluted to 1/4 of recommended normal dose. There's plenty of water in the propagator, but the cubes are set above the water.
Rockwell and peat pots are really not conducive to healthy seedlings. I would suggest that at the two true leaf stage transplanting into small pots or cups with drain holes in the bottom using soilless potting mix. I would also remove from the closed system and put the lights within an inch of the tops of the plants. The lights can be on for 16 hours and off for eight. Lots of info on this and other tomato sites.