I would advise against the nutes (sorry pollack), simply because your roots can easily be burnt. Honestly it looks like your soil is too moist, and its not getting enough air. an 80/20 mix seems VERY lite, the only reason you add perlite into the coco coir is JUST to bring air in, the more perlite, the more air.

I've got a bean I popped 4 days ago, almost the size of your little sprouts now I think your over watering, OH and make sure you have drain holes in your solo cups, you don't want that nasty water sitting there - I typically have to hit my solo cups every 3-4 days with about 1/3 cup of water each (when I do soil grows)