I really love the ground cover of little plants. Did you say there is 15cm/6" of activated carbon in your filter? Some AC can add phosphates to the water. Here is a link to a page talking about it https://www.practicalfishkeeping.co.uk/ and here is a second https://fishlab.com
Its only some kinds of AC I think the one made from coconut shell is preferred. I personally only use it to clear medication or tannins as needed.
As a side note using AC all the time may be linked to hole in the head disease. This is in salt water I am unsure if it even happens in fresh
www.thesprucepets.com
I would test for phosphates it see if you have elevated levels.
Do you know the name of the ground cover?
Its only some kinds of AC I think the one made from coconut shell is preferred. I personally only use it to clear medication or tannins as needed.
As a side note using AC all the time may be linked to hole in the head disease. This is in salt water I am unsure if it even happens in fresh
www.thesprucepets.com
I would test for phosphates it see if you have elevated levels.
Do you know the name of the ground cover?