I live in Los Angeles and we have a number of importers here. Reducing initial losses/casualties is a big part of their philosophy when it comes to their system design and techniques. Most of the importers I've been to have massive systems with very large filtration systems including sand bed filters, large protein skimmers, UV, huge pumps with very high flow rates through their systems and last but not least, they do very large daily water changes. They generally have seperate systems for fish and inverts. The fish systems are typically run at low salinity levels around .014 to reduce stress on the fish, reduce the spread of parasites and reduce costs due to the volume and frequency of the water changes. This unfortunately can increase the casualty rate once the fish leave the wholesalers as many stores don't practice proper acclimatation techniques and go from really low salinity levels to more moderate levels too quickly. They don't match the salinity of the incoming water like they should and fish often suffer from osmotic shock and die within weeks as a result. Some wholesalers/importers use mixed salt while others use natural ocean water which they further dilute with fresh purified water to keep salinity levels down.
That said, a truly heavily stocked system would typically be around 1/2" of fish per gallon of water volume. Because of the life support equipment in those systems and the volume of water changes which can be as much as 50% a week, those rules get bent. In a typical display tank, 1/2" of fish per gallon of display volume is typically accepted in the industry as a heavy load.
