Interesting.
Here is what I have done today:
I used an bladed algae scraper and scraped the bottom of the sump and display. It is a bare bottom display and this weird layer of "I dont know" has apparently formed. It was thick and kinda brownish green. It came off in big thick sheets that looked, honestly, almost exactly like nori sheets. I would just say its solidified detritus but I have A LOT of flow in this tank (x4 MP60 + x2 MP40) that generally keeps any free floating stuff suspended until it goes down the overflow.
After that I did a big water change (60 gallons) and sucked out as much of the cyano and gunk that I could (sump is lookin' very nice after this).
I then corrected all of my parameters to where I want them ideally or where I think would be beneficial. I went a little overboard with the Nitrate and Phosphate but I am not terribly worried about it.
Alk: 8.3
Cal: ~400
Mg: ~1500
NO3: 20ppm
PO4: 0.29
I am going to leave all the dosing offline for tonight and see where all the levels are tomorrow at this same time. That should at least give me a window into how much this tank is using.
Few side notes: Underneath that layer of gunk on the bottom of the tank there were several places that looked like coraline algae has begun to grow but stalled out (they were white).
I did as
@bdare has recommended and used Bicarb instead of the soda ash for the big alkalinity correction. If the tank does indeed respond favorably to this I will switch over to that for regular dosing. The pH boost from soda ash is nice but with the CO2 scrubber (even when not dosing alk at all for days) my pH stays pretty stable between 8.1 and 8.30.