18000 GPH Pump? That sounds like way too much flow intially for QT. I would turn it down as it's causing your fish to burn a lot of energy.
Agreed. I’ve caught, held, raised and sold thousands of Achilles and we would never quarantine this fish with flow anywhere near this amount.
Cut the flow down considerably, turn the 40’s off. Aerate, offer some nori to browse on. For copper I like Mardel copper safe as it’s soft on Achilles, be careful with Seachem. I wouldn’t offer live food or frozen until after the fish is nibbling on algae. Otherwise you’re just making mess that will foul the water and have you sticking your arm in tank to siphon up uneaten food thereby stressing the fish out more. You can put them in high flow once they’re strong and healthy in the main display.