its rare/unusual to see a loss cascade that extends out over several days/couple weeks, a true crash is cloudy water and near total loss, fast.
Im not versed enough in fish disease/prevention stuff to know how to prevent a recurrence before you restock up with new fish, that's worth inspection for sure before continuing on/restocking (to make sure round 2 is following today's best protocols for fish disease prep, fallow etc might be required)
after that assessment we'd have to rule out any type of contamination, copper contam somehow getting over into the main tank, any sourcewater issues/mixing stations...all that needs to be verified not causing problems before a smooth reboot in my opinion.
a 10-20% water change wont help anything if we're dealing with those type issues so am really hesitant to offer partial works on this large tank, it seems like we'll just set it all back up and repeat somehow/want to prevent that headache.
*Im big into pre modeling actions now in reefing vs guessing...so much of what we do is guess, cross fingers, see if something lives. so if I was to pre model something in order to give your big tank a chance on reboot #2 it'd be this:
set up a nano model using its parts. this eliminates issues of contamination as a nano will kill things fast when under stress, if a smaller easily controlled nano reef stays alive using your current water preps etc/materials from the tank in question then we've removed some guesses by pre modeling. you don't have to downsize forever, downsize until you feel your rocks and sand are safe, then begin again after fish protocols are checked off. the way you'd pre model it is to keep the current large tank running with topoffs etc, no big changes.
take out a section of rock and scrape off all the algae with a knife like its a dental visit and you're cleaning out reef teeth using picking motions like they do on our actual plaque. be detailed, pick out algae, rinse in saltwater, clean off a large section of rock ready for use in the nano and put it in the test bed with all new mixed water. leave the stuff in your current tank/clowns and snails.
go get a handful of snails and crabs, new, from fish store and put them in nano along with your rock. simply top it off, hardly feed it so you wont overdrive it, and see if they're alive in ten days or so. if they die fast (no reason to) then there's some contam issue we haven't seen yet.
of course this is lots of extra steps, but testing things in your own tank wont highlight anything we can see. prepping the side test and using animals in all new water, from lfs, isolates just your water prep and rock as actors in the test. its ideal and cheap.
if they live fine, showing your rock to be no problem and water mix to be no problem, then we can move on to prepping your big tank for a nice reboot, which is basically all new water, new sand and the same rocks.
we'll know your animals wont die because they didn't die in a nano.
Ive read in some places that copper treatment absorbs into tank glass...has that display tank ever been treated for fish disease directly