Help with a few seedling problems!

2 out of my 5 seedlings have started to curl their leaves downwards and I have a few ideas as to why but I would like some clarification. They're currently in a plastic tub with CFLs and I have ruled out over and under watering as my root growth has been thriving amongst all 5 plants equally. I was wondering if it was possible that it's due to then being exposed to temperatures as low as around 50-60 degrees F during day 4 and 5 but only during the night hours. I, just today, installed a small heater which keeps the area around 80 degrees F and I have a circulation fan. I'm hoping it was just the cold and that I've fixed it but incase it's caused by something else I would like to hear what to do about it before it's too late. Also I was wondering about this purple stem. I know the cold temps can cause some purpling but could there be something else going on? Again they're only 6 days old. Any feedback is helpful!

I would not worry about the purple stem, this could be down to genetics. The lower temps wont help but wouldn't have thought this is the issue. Notice you have them in uncovered transparent cups, roots will not like light so cover them with something to keep light out but this will not be issue either. Have you overwatered, this I would think will be the cause, wait until top of soil is dry before watering, I know you don't think it is this but that's my guess. Other than that have you ph'd your soil and water.

I'm going to just throw this out there. Are there drain holes in those cups?
Don't kick me in the nutz for asking, it happens.

I must agree with you D Greezy ! This just happened to exactly 2 out of 10 of my seedlings ! I just thrown them today ! Unfortunately a cold windy night this friday got them suffering !
I would keep these two of yours under surveillance for some time ! Hope they do well ! Cheers !

I vote overwatered.

Personally I would top off the cups with soil and bury what stem I could. I don't think they are excessively stretchy but it will help.

As Ring60 said, Does the excessive water go down easily ?

looks like soil too wet. use a mist bottle on seedlings to keep the soil lightly moist and not wet. as mentioned above, make sure drain holes are in the cup. GL

Turns out y'all were right, I had made drain holes on all the cups but that one and as soon a I did some water started dripping out. Thanks a lot! But one question if the tips of the new growth just got slightly burned will it continue to grow or should I be worried I killed the seedling?

Haven't watered since I posted the pictures and this morning the soil was dry on top and I didn't have time to really check them fully but I will soon when I get home. The leaves don't seem to be perking up, although this one seedling is just slightly outgrowing the others. Could the problem be something other than over watering?