Seemingly healthy fish dying after being moved from quarantine?

Hello, I've been having an issue for almost a year now where after being quarantined for 4-8 weeks the fish die usually within a week of being moved to the main tank. This has happened for the last 5 fish I've gone through: two koran angels, a clown trigger, a copperbanded butterfly, and a nasa tang. During quarantine I've treated with hyposalinity or chloroquine. The pattern has been pretty much the same, the fish will eat fine for a few days, then stop eating and hide in the rocks, to be found dead within a few more days. The only time I've seen any kind of parasite or infection was one of the koran angels showed some slime on its body about when it stopped eating.

All of these fish appeared very healthy and were eating fine while in quarantine. I am always very careful not to stress the fish when I transfer, and exchange the water in the bag over a period of 2-3 hours at least. I've been using a net to catch from quarantine and think I will try using a bag next time to see if the net is causing too much abrasion of the fish.

Water quality in the reef tank is perfect as far as I can tell, except it had been a little low on hardness (6 dkH) and I'm treating that now.

Any thoughts? Aside from not using the net I'm totally stumped. Could it be that the heavy flow rate (2 mp40s for a 120 gallon tank) is too much for these fish that aren't normally considered "reef" fish? Another crazy thing is that there are fish in the display that have been there for years and none of these longterm fish have been dying, so it's not that there's a toxic condition in the display.

I have a clown trigger that's been in quarantine 4 weeks and will be moving him out in 2-4 weeks so I really hope people can give me some insight as to what is happening here.

Thanks!