I would say the two possible suspects was elegance and reactors. Those are the only two things I can think of.......
They are both easy to check.
Elegance will probably be the fastest to see a result in the nem. That is if the clowns move back and forth constantly between the nem and the elegance. If they don’t move back and forth constantly, I highly doubt it could be an issue. Especially as you had a mass die off of the other nems. But it is something to check.
If they do and the elegance is actually annoying the nem via clown contact, you can move the elegance to another tank if you have one. Or you can build an egg crate cover for the elegance to keep the clowns out of it. You should see a response from the nem in a day or so if this is the problem, provided the nem has not suffered too much.
The reactors would take longer to see a difference, especially if they stripped the water to fast.
Just a note: Reef roids is high in phosphate. I dose it as well, but a little goes a long way. Nems also do not need to be fed. They will end up with residual and sometimes the clowns will feed them.
Just remember, either way you need to take things slow. Bubbles are tough nems and can come back from a lot.
Personally, I would try the egg crate over the elegance first, and move it to the sand bed, so it is easier to cover up. A little less light will not hurt it for a short time. Like I said, you should see a reaction in the nem quickly if this is the issue.
It does worry me that nitrates are 0 though. Mine hate it when nutrients bottom out. (I have to dose
No3 and po4).