Sure does seem like something is leaching (or accumulating) in your water. It was corrected for a while with massive water changes, and then came back. You could try the massive water changes again, to see if again that fixes it, at least for a while.
Meanwhile, would keep testing your water. Not sure if our standard copper water tests are granular enough at the low range, but might be worth doing if you have them.
Alk swings are not good, for sure, but hard to believe it would cause RTN to that extent though, and so suddenly. I have had swings between 7 - 9, but no effects that I could see.
And hope you are past that hydrogen sulphide stuff with your pellet reactor. I definitely don't like the recirculating types. As others have said, would ditch that for a while, too, until you can figure out what is going on. Your higher phosphates are not good either, but again, shouldn't have caused the sudden RTN, I don't think.
Anyway, keep us informed on what you find. We all want to learn from this, if we can.
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