[mistergreen]

Yes, you can freeze mud and insert under your substrate. I would use regular potting or top soil.
Dont forget to change the intensity of the light as well when you don’t have co2.

 

[mistergreen]

Your plants and lights are fine. I wonder why the H. polysperma didn't grow. That plant is a weed.

 

[mistergreen]

I've removed the CO2 injection three weeks ago, maybe the plants need more time to adapt to the new condition

Yeah, plants will grow significantly slower without CO2. You just need to wait. Well on the good side, you don't have to trim so often anymore.