[Help with Amazon Sword]

Those are new leaves at the bottom with black all over them. Thats why I'm concerned. The stems going up are the old leaves that are melting. Expected that to happen.

[Help with Amazon Sword]

[Help with Amazon Sword]

[Help with Amazon Sword]

Ive had this sword for about a month and a half. It started to melt back, which was expected. The new growth leaves were sprouting out and growing extremely well, until about 2 weeks ago (1 month in tank). I suspected it had already eaten all of the root tabs (2) I placed underneath it. So, I added to more directly under it. 

Now the plants leaves are looking black, but it doesn't look like the black beard algea I have seen. I also watched all of the nutrient deficiency videos, and this doesn't seem like any of it unless I'm missing something. l

I use API root tabs, dose 3 ml of seachem flourish every 3 days, and api c02 booster 4 ml every day. Water is 76-77 degrees, GH ~75 ppm, PH 7.0, near 0 Nitrates/nitrites. 36 gallon tank. 

I have a 27 w led about 9 inches above the tank, assuming each light is 1 watt?? 18 inch deep tank.