Sorry to hear. He's been living poisoned for 20 years by his own waste, I am sure, a 43" paroon in a 75 gal. I think this may be the plausible explanation for his quick demise in good water: either he was already at the end of his rope (coincidence or rehoming stress pushed him over) or he failed to adjust to good water, his whole organism had adjusted to live and survive in filth and there was no coming back, unless maybe extremely slowly.
It's been known to happen - animals dying from being brought out of long, strong abuse into good conditions.