I have had a consistent problem with leaves curling since I moved to my new place. I also have had problems in many places (though in the same area of the country) where my stems turn black, then separate. I only have a few stems with that problem at the moment and they just wouldn't show up in my pictures.
I had a past thread where someone identified it as a calcium deficiency. I don't have a Ca test kit, but I started using Seachem Equilibrium as a calcium source. I got that advice for the lack of color in my plants originally, then again when I posted about stems turning black and the tops (and sometimes in the middle) separating from the plant. At that point in time, I was not using a TDS meter. I would do two table spoons. I found on occasion, I would loose a fish during a water change, especially those who like lower PH.
My TDS meter fell in the tank so I need to get another one, but I found in general, this raised my TDS about 300ppm so I felt that was too much. These are some tests that are typical for me, given what I have to test with my kits:
My parameters in tank are:
Nitrate: 10-20 (correction)
PH: Maxed out on both pH and high range pH (without CO2)
KH: around 11 degrees
GH: around 8 degrees
Out the tap:
PH: Maxed out on both pH and high range pH
KH: 1 to none, hard to test
GH: 8 degrees
I know other people have had leaves curling but it seems the black stems don't happen often. They don't happen to me when adding Equilibrium, but I feel I may be adding too much, maybe there is a better solution here.
I don't have much algae but I have a little bit over everything at this exact moment. A tiny amount of BBA, a small amount of diatoms, green dust on glass but not major, and a bit of staghorn. None concerning or overbearing.
Other note, I get BGA like crazy whenever I start a new tank. I just treat it with EM and it typically never comes back. I only mention it because this also seems not typical. I mean every tank with CO2 I have ever done. It almost never comes back.
I am just hoping to find a solution. I believe the lack of color is associated with this because I can get great colors in my low tech tanks that are not low tech plants, I can get great color in my high tech by starving them of nutrients, but that is short term.
Here is what the curls look like, I will try to get a picture of a black stem when it is in a place more easily photographed. Notice how some leaves have the "bottom" facing the surface.
EDIT: Sorry for the sideways pic, I don't know how to fix this. If I rotate it on my phone, or in other software I have in Windows, it stays sideways. Also, a black stem pic I found, which is sideways as well: