I would look at other things from what you describe. SPS corals are certainly your first indicator for alk related problems, and if your SPS is fine then I don't believe you have any alk issues.
Zoas.. any chance of pests? Zoa eating nudibranch's.. maybe a fish? Sometimes zoas act like xenia and grow great and then all of the sudden just melt away for no apparent reason. Mushrooms.. flow or light related? They like low of both. Most things will not even think about eating mushrooms so I don't think that would be pest related.