[Canadian]

I agree that you should look in to lowering your alk if you're using Zeo (but if you haven't been using Zeo since you started trying to keep SPS then this won't explain your problem).

Again, check your SG with a refractometer - not a "calibrated" hydrometer. You can't believe the number of times people have assumed their "calibrated" hydrometer(s) were accurate only to find they either went bad or were never good (Brad can attest to this). The problem may not be that the SG is 1.022 but I would be surprised if it actually was 1.022.

Flow coming from the sump "at some astronomical rate" doesn't tell us the flow rate. What return pump are you using? What is the rated flow for the return pump? How high up does the pump have to pump? How many elbows are plumbed into the return pump plumbing? Those Koralia 1's are inadequate in a 75g.

I would stop assuming your Mg is ok. If you can't get a test kit locally order one from J&L or Ocean Aquatics - they're both close and shipping will be relatively cheap.

When was the last time you measured the TDS of your RO/DI water?

Have you ever added any detrivores to your deep sand bed? I would guess your deep sand bed is a sewer dumping nutrients back into the tank as evidenced by the fact that you continue to have hair algae. You 100% do not have zero phosphate if you have hair algae growing in your tank.

So again, I implore you, if you want help figuring this problem out take a picture of your tank and post it here so we can stop guessing and see what we're dealing with. Trying to "diagnose" your problem without seeing at least a picture of your tank is just going to beget more guessing.