The long stems with the 'feather' shapes leaves are emersed leaves. You need to take them off now. Slide your thumb down to the base of the root and push until the lead breaks away. I have that same darker vein thing going on when I had my swords. Seems to be prominent in early submerged leaves then disappears as they grow. The new sword leaves will appear thinner. I'd add the co2
Here you can see the same thing on my leaves.
Here you can see that it did not matter.
Remember as the plant grows larger so does its appetite for carbon and nutrients.