One thing i might suggest is that the rocks you are using are not neutral rocks meaningthey will increase the Ph of your tan k, possibly affecting your ember tetra
Why would you say that about the embers? That are all happy and colorful and content. And my pale ones are even coloring up from their original state of being almost colorless. I also didn't think that ohko stone was a stone that increased hardness. I guess it could though, it is pretty soft after all.