I have some very strict rules for buying marine fish that will save you a lot of grief.
*Only* and I mean *only* buy fish from fish stores that have very low death rates, and have nothing but healthy fish in the store, and will also be willing to show you the health of the fish by offering them some food. Healthy fish will eat like pigs.
Also, even though you see some pretty fish in the store you've got to have, ask how long it's been there. That fish you are looking at could have been in a cargo hold in a plane the night before in a little plastic bag getting ammonia shock, or been in that display tank for two months and begging for somebody to take him home. I *never*, EVER buy fish unless they've been in a fish store for at least two weeks from the time they were brought in. Gives them a chance to get over shock or show signs of disease. Good pet stores will even let you put a deposit on the fish and mark it as sold.
I used to buy fish from a local mom and pop fish store, until I got tired of 50% death rates. Seems they not only sell fish that have recently been transported, they keep low salinity levels to keep parasites down. Once fish are moved to an average home marine tank, they are always in a state of shock.
The store I buy fish from now is actually one of those pet warehouses, but with a very ethical staff. They won't sell new fish, and they keep their water parameters at typical tank levels. The result is I've never lost a fish from them, and all have migrated to my tank with little effort or stress.