Ouch.
Having lived through several inexplicable SPS die offs, I feel your pain.
Like I've said before sometimes it's not even really a testable parameter. There's so much stuff at play, when an event happens, our attempts at an investigation can sometimes just be the Texas sharpshooter syndrome. You hope whatever it was is resolved and hope for the best sometimes.
But, like you hinted .. onwards and upwards. Your tanks sure have a good look to them still.