Chronic low PH and inconsistent ALK

[waynemah]

I’ve always had a PH *issue* with my tank. I realize that this is one of those parameters that might be a “new tank” thing, or it might just be the PH it wants to be at, but there are some other un-answered problems that could be related.

It’s twice now with two different salts that I’ve had my ALK drop out, once with Reef Crystals and once with Reef Salts. I don’t believe it’s the salt itself, but my tank seems to consume or precipitate alkalinity. Hopefully, someone can help me piece this together to find the cause.

Water parameters:
Salinity: 1.026 on NSW calibrated refract, 53mS on profilux.
Calcium: 410
ALK: 7.5
MG: 1490
PH: 7.85 - 7.95
Temp: 26C

Other parameters:
Tank up for over 6 months.
Sump Light on reverse cycle.
Effluent from the calcium reactor, this reads 450 CA and 8.4DKH (does this sound about right?)
Was using Reef Salt, 420CA, 7.2DKH, 1280MG and 8.4PH.
Now about to switch to Reef Crystals, 410CA, 11DKH, 1300MG and 7.9 PH.
PH does NOT raise more than .1 when aerated outdoors (although there is a temp change… Likely not a solid test).

Related equipment:
Calcium Reactor with course ARM (high magnesium), 6.8PH.
BRS Magnesium Doser (currently off to bring the number down to lower 1400's)
RO/DI with Tunze ATO.

Recent Events:
Changed to Reef Salt for higher PH with RO/DI to try and resolve PH, ultimately lowered my DKH and seemed to lower PH overall.
Alkalinity drops faster than Calcium, PH also drops when this happens. No clams in the tank.
Tank has some diatoms on the sandbed with a hint of cyano (might be unrelated, not sure).
Changed all RO/DI about a week ago. No change since.

Suggestions on the table (from other people):
Magnesium is too high, taking up “dilution space” causing the alk to precipitate. – Not sure if this is possible???
Add a Kalkwasser solution to a reactor to raise the PH and ALK, have it tied into the ATO. – I’m somewhat afraid of this and it adds a level of complexity. What is the best way to do this?
Change salts to a higher level of ALK and PH (H20cean?) – I’m ok with this… What do you think?