Sorry, but IMO - try not to get overwhelmed (specifically by the post above). SPS = stability promotes success.
Assuming you have not dosed any supplements or other things which could have drastically altered your levels (alk, ca, mg), I would work at dialing in that calcium reactor to maintain a DKH of 3.5 (+/- 0.5), once you have it set (based on your current SPS load), memorize how you did it and leave it alone for a month or so. Check DKH every other day until stable. Chances are your tank improves after a water change because of the increase in DKH, again, JMO.
I have never checked Potassium, have never experienced Ph issues, refugium issues, salt issues or ventilation issues. But that is just my system.
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I'm out. 
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