Help, my rock is dying

[pshootr]

This white substance is also visible on the sharp edges of his rocks. This leads me to think that water flow is causing whatever this is to accumulate in isolated areas such as sharp edges or where the flow is strong.

 

[pshootr]

It’s probably just the shell bleaching. I think just to be safe I will stop dosing until I have proper testing equipment.
Thanks for all your help!!

Definitely don't dose magnesium anymore until a you have livestock which will deplete it, and also until you can test for it.a teaspoon of magnesium per week sounds like a ridiculous amount to me. Magnesium is much of what keeps calcium and alkalinity in balance. I would probably do a 50% water change for two reasons. First reason to remove a lot of that magnesium, and the second reason to replace any calcium and or alkalinity you may have lost due to the high concentration of magnesium. I hope one of the chemistry experts times in on this one.