It may not be anything you are doing. A lot of the problem is how the fish are collected. When I get fish I ask how long the store has had them unless I check every week looking at the new shipments. A QT tank is very a good idea so you can see if the fish is eating and if it is eating but still loosing weight. Also always check the LFS
water before you start acclimating the the fish. The store where I go their water is 1.020 I keep mine at 1.024 so I take my time to drip acclimate them.
When I see how the fish are shipped then handled at the LFS
it makes me wonder how any of them live to get into a home aquarium.