How to select a Powder Blue...a tough fish to keep. Make sure you follow this as outlined in selecting your Powder Blue...cut and paste..
Selection:
Bad specimens of A. leucosternon are easy to spot. Most have darkened blue body areas, perhaps with a white "stress" bar, torn fins, and other evidence of accrued shipping and handling damage. Decent specimens are harder to assess. These all have the quality of "brightness", that is, clear eyes, constant movement and an awareness of your presence. "Spaced-out" specimens should be left.
Index of fitness is a fisheries term for the "fullness" of fish bodies. It’s quantified as the circumference of an animal divided into its length. Specimens with a higher value are obviously fatter than ones with lower indices. You want to select for a Powder Blue that is not too thin, particularly in the upper body area (the flank, up and behind the eyes). Skinny specimens rarely recover.
Some "flashing" or scratching is to be expected of all specimens, but this should not be excessive. Nor should respiration be labored or too frequent. About sixty gill movements per minute are what you are looking for.
Newly arrived specimens are better to sort amongst other than longer-on-hand ones. Like most marine livestock, and particularly Acanthuroids, Powder Blues are "starved out" for a few days ahead of shipping to reduce in-transit pollution in their shipping bags. Unfortunately this starvation can be persistent once the animals are received and shipped through wholesalers, jobbers to your LFS or retailing supplier.