Newbie question about health of seedlings

Is there a soil shortage in your part of the world? Next time use decent sized pots and fill them right up, those tiny cups and only being half full, is basically torture for your seedlings and is limiting their potential size. A plants' roots will be about twice to three times as long as the plant is tall, so you can see that your plants roots will already be out of room and this can lead to stunted growth. Black plastic pots about 4 inches across and 5 inches deep are much more appropriate for seedlings. The tip damage has nothing to do with humidity levels, it is most probably the result of the leaf hitting the side of the tiny cup. In fact, unless you live in a blazing hot desert, humidifiers are somewhat irrelevant. Remember too, no plant will ever have 100% perfect leaves, 100% of the time. Also, I hope you cut holes in those tiny cups for drainage, otherwise you run the risk of drowning your plants, one of the symptoms of drowning can be downward curling leaves.

They're getting nute burn because you've started feeding them awfully early and if, as Mrsm1th suggests, your soil has nutes in it as well, then yeah - it's definitely nute burn. Just give them pH'd water for the next week or so... and make sure the solo cup has some hole poked in the bottom so you don't end up overwatering them. As for humidity, an easy way to increase the humidity in your tent is to soak some towels in water (wring them out so they're not dripping) and hang them from the top bars.... works a treat... just use as many towels as you need to get the humidity where you want it. You didn't put a whole lot of soil in the cups so your roots are going to be jamming up with no place to go VERY quickly... by the time the seedling is about an inch tall, those roots could be 6 inches long - so plan on transplanting soon! Good luck!