To better understand the situation, could you please post a clear photo of your entire tank, including some closeups of the areas you think are relevant? This will give us a better sense of the plant load and overall setup.
Regarding the water parameters, it would be helpful to know the current levels of GH, KH, and temperature, in addition to the pH, which you mentioned is 6.8. Additionally, what are your current nitrate levels?
It would also be beneficial to know more about your lighting setup. If possible, could you provide a link to the manufacturer's specifications or provide some details on the spectrum and other relevant characteristics?
Why are my plants turning black?
Re: Why are my plants turning black?
I'd say ditch the wonder shells, they're not necessary. Those plants you've got in there, they don't require 10 hours of light, 6-8 hours should be sufficient. What's the tank size, by the way - gallons or liters?