I cant quite tell how many sets of leaves you have but if its under five, flush'em now as you shouldn't be ferting until you've got atleast 5 leaf nodes. I say this because it looks like nute burn to me.

Disclaimer, im a total noob when it comes to growing, so take everything i say as just a noobs opinion.

edit: looks like you posted some more photos whilst i was typing, so my theory on it being too early to fert is out. Still kinda looks like nute burn to me.