In my experience, substrate can be good for bottom dwellers in quarantine. Just a light substrate. It helps keep away any nasty that may form from uneaten food or a dirty tank even with wc/cleaning. I quarantine everything I get, to be safe as it's better to have the new fish die off (even if they were expensive) than the old ones in the main tank. Plus the qt tank is easy to clean, observe for any parasites/diseases, and medicate. I even tell my buyers to qt fish they get from me. Don't want to spread anything. If this has happened from the same LFS with other stock, I wouldn't trust it even though they say they qt the fish. Best to figure out what is killing your new fish before buying/adding more to the tank.
Where they non-active when you bought them or did this just start today? Was the fish flashing against rocks/plants?
