In the past 3 days I've noticed yellowing of some of the lower leaves and spots on them, most are dark some appear to be almost clear - like the green has been taken out of them. In some of the worst cases the edges of the leaves have shrivelled and are dead. They are 15 tomato plants pretty close together under 4 T8 lights for 16 hours a day. They're in 16oz dixie cups with holes in the bottom planted in miracle gro seed starting mix. They're 6 weeks old and I've never fertilized them as the miracle gro says it feeds for like 3 months or so. They were very dry when I noticed them, so I immediately watered and it doesn't appear to have gotten any better, but it also doesn't appear to have gotten any worse (at least noticeably). They looked great and in 24 hours a lot of the leaves had spots and were yellow, so I'm hoping since it hasn't really gotten much worse it was just lack of water. I got early blight realy badly last year and lost all my tomatoes so I'm scared/paranoid. Sorry for all the verbage. :)

Is this something I should worry about?