Hey everyone. I figured I would jump on board and share a lil of my experience. I am successfully breeding cardinals and my water parameters are as follows:
pH 8.0
TDS <350ppm
using RO water that soaks in coral chips before replacing.

I know of those breeding cardinals in TDS as high as 800 ppm.

I also supplement using montmorillonite powder once every two weeks.

Cardinals require an extremely well aged/cycled tank rich with biolfilm. Hope this helps