When shopping for a Harlequin Tuskfish, make sure it has clear clean fins and is actively moving about the aquarium in search of food. Be sure to see it eat! A Harlequin Tusk always has room for one more bite of food! Specimens between three and five inches are the ideal size. Tusks less than two-inches in length are fragile. They have a high metabolism and insufficient reserves of fat to withstand a period of prolonged starvation. Tusks larger than five inches dont ship as well and often have a harder time acclimating to captivity. I also recommend spending the extra money and purchasing Harlequin Tuskfish that originate from Australia. It may have something to do with the way they're captured, shipped or handled, but Australian Tuskfish have exhibited better survivability than specimens originating from Indonesia and the Philippines, especially the smaller tusks!
As with any new fish, its best to quarantine them become introducing them into a display aquarium. This give the fish a chance to acclimate to captivity, become used to your food and feeding regimen, build up reserves, and be treated for disease if necessary.