Has anyone any experience of using the red substrate, as when I stir the black gravel layer which is above the red substrate in my tank, I'm getting quite a lot of gas bubbles appearing. I am quite tempted to remove some of the red substrate and mix it with clay and add the black gravel layer on top of that. I've had my tank set-up for approx. four months now and I've always had this problem with the gas bubbles. I would really appreciate any advice or suggestions as to how I solve this problem.