I needed to QT ALL the fish then why not just treat in the display tank?
Copper will kill your inverts or possibly future inverts. Mosy/everything on your LR
will perish also.
Same thing with hypo, except it is not a long term problem. If you treat with hypo, your LR
might perish along with the inverts and this could cause you another cycle. All the die-off will produce lots of ammonia, then nitrites, both toxic to fish.
It would be impossible to catch all the fish without removing all of the LR.
You can build traps and get them out that way.