Deep in the live rock and the live sand probably does the job in my tank, based on your information. I stopped measuring when I stopped getting readings.
I still check calcium levels, though less often, to make sure my calcium reactor is dialed in.
I have a downdraft skimmer about twice the size of the one in your picture. I wish I could hide it somewhere.
It's far to clean under that tank in your picture. I have dust, salt creep, tools, fish bags...