Did the KH drop happen right after the WC? If so, your tap has less buffering capacity (KH) than it used to and the WC diluted the tank's KH and caused a downward pH swing.
If that's the case, this is what I would do:
Put the fish in a small container, like a 5gl bucket half filled with tank water.
Fill a separate container with the same amount of water and add baking soda to get the KH back to 9.
Add the re-buffered water to the fish container slowly, like a glass every ten minutes, occasionally remove a glass or so of water from the fish container to keep the water level the same. Do this over the course of an hour or so and the fish container will eventually be 9dKH as well. The longer you take, the safer the parameter change for the fish, true they are dying, but too rapid of a change will be excessive stress. Fish generally acclimate faster than inverts who breath through their skin and only an hour or two is usually sufficient for inverts.
In the meantime, you need to bump the tank back to 9dKH with baking soda. Be careful with your baking soda additions so as to not bump the tank up too far, if you go too far, add tap to bring it back down. It would help to treat all of your fish as stated above, so that you can bump the tank immediately without issue. Generally, I would say just leave all the fish in the tank and bump it up slowly, but it's a large tank, and precise buffering over time will be easier in a small container and likely require a few less KH tests on your part (Otherwise, if you already know how much baking soda it takes to bump your tank's KH to a few tenths of a degree (6.4 to 6.6) then you could just leave all teh fish and buffer the tank back up over the course of a few hours). Once you have the KH of both the fish container and the tank back at 9dKH, it's safe to put the fish back in the tank and depending how far gone they are, they may start acting normal pretty quickly.
In the future, you'll have to be on top of your change water's KH and make sure it always matches your target of 9 before it goes into the tank.
Hope that helps, pulling for the fish!