from the look of the leaves and this chart, it looks like Potassium

Thanks for the reply. My plant is already looking a lot better since giving it nutrients, and I think now that you say that it was the potassium in the flowering solution which helped it, although now my ppm is at 500 which is a bit high for a plant of this size. Check out the progress:

What do you think?

my toms are all at 550-600ppm and doing just fine, they were started 3/21. I read your other post and you really need to not worry about them so much at the beginning unless they're not growing or if they're falling over/dying off. Some of my leaves got a little damage from my LED's when I removed the dome but they're recovering just fine.

Do you know why my leaves are so much bigger than the average tomato plants? One of my seedlings which I culled had skinny leaves just like yours, but this healthy one which is taking off as you can see has fat, nice dark green leaves unlike most seedlings. Odd...

Genetics? Just like you and your mate could have two kids, both boys. One might have blue eyes, one might have brown. One might grow to be short, one tall. If you hadn't have culled the other one it probably would have caught up or even surpassed the one you have now. You just never know. To early to really tell what's going on with them when they're really small like that. This is why you see 4 plants in one cube, who will be the better plant? I don't know and I won't cull any of them for at least another two-three weeks.