Hi Pablo,
Your LFS is right and they may be diffucult to keep. On the other hand if you consider some bullet points highlighted below keeping them is easy :
1- Find a reliable LFS buddy.
2- Before buying check the origin of the Powder Blue( Singapore or Indenosian origins are generally hardy)
3- If you can check with LFS buddy how the Powder Blue caught. Cyanide caught fish die quickly
4- Ask your LFS to keep the powder at his tank a week without light and ask him to feed the fish. If you have a quarantine tank you may do the same.
5- Before buying check the powder blues moves and activity if the belly of the fish is outward it means you will have time of accaptance of dry fish food.
6- After buying as a first step acclimate the fish by dripping method. Acclimation without dripping by put this fish into stress. Make the acclimation time at least one hour.
7- If you have other tangs especially with the same shape of powder. You should take the others out or you should NOT buy a powder tang.
8- If you want a guarantee of success I do strongly recommend introducing other fish with Powder Blue at the same time.
9- Once the powder blue starts eating it means all in order. I recommend garlic mixed granule food.
10- And the last but THE MOST IMPORTANT is do not cause any STRESS on this fish. They are very delicate to ICH. For example do your aquarium maintanance after all lights off and during the powder is resting.
Hope all helps and good luck.
brgds