Browning spot on one leaf

So I have a plant that is 6 days old. It is currently in a DWC bucket with a little less than the recommended dose of solution.

One of the leaves has a brown spot forming on it and it looks like it is dead in that spot.

And in my solution bucket, there are crazy ass little black things in the bottom right corner. Anyone have a clue as to what these could be?

Sorry about the shitty quality of pictures, they are from a webcam.

Thanks a lot guys.

So here are some updated pictures, hopefully these might be able to help.

The problem seemed to be getting worse up until I changed my water earlier this morning, then it settled down a little it looks like.

Help would be greatly appreciated, I fear for my young one's life.

Any ideas?

it looks like a nutrient burn. don't fertilize when you water next time.