I grow Xenia and sell to a couple LFS's (they say it ships poorly ) Keep useing carbon it is not the problem , get an iodine test kit if iodine is low that could be the problem .You also need stable ph . It looks as if you have it in the bottom part of your tank .
1. How deep is your tank ?
2. What type and wattage of lighting are you using ?
1. How deep is your tank ?
2. What type and wattage of lighting are you using ?