I know I will probably be flamed for this post but I just thought I would drop my $.02 in and play devils advocate. I have had much greater success in my fish-only breeding systems at a S.G. of 1.018 than with full strength S.W. Keeping a reduced salinity is in fact a key feature in inducing spawning. The larvae seem to respond especially well to lowered S.G. In side by side trials in my lab, I noted a 25-40% increase in survival and an 18-30% increase in individual growth rate in those larvae kept at a depressed S.G. Oxygen saturation and parasites did not play a role in the results, rather it appeared to be due to a decrease in metabolic workload by requiring less effort to maintain the internal salt level. Numerous other studies have also shown an increase in juvenile and adult growth rate and survival in marine fishes kept at lowered S.G. I believe that the argument that the environment in which they are found must be the best one is an overly simplistic view. There are many examples in nature of organisms that adapt to environments because habitat is available rather than because ideal conditions are available. A good example is found in the halophytes (salt tolerant plants typically found in estuaries) and mangroves. 70-90% of their metabolic energy is devoted to expelling the excess salt. Growth rate was increased dramatically when grown in low salt or even freshwater conditions. In the case of fishes, the ocean provides an abundance of habitat and it is believed by many that most marine fishes are evolved from freshwater fishes. So this may be a case of coping with seawater at 35ppt, instead of actually preferring it. In short, I don't believe that keeping marine fishes at decreased S.G. is intrinsically wrong, in fact it may even be beneficial. Rather, in the specific topic posed on this board, I think the real question would be whether a series of rapid S.G. changes from the ocean to the wholesaler to the retailer to the hobbyist would stress out the animal additionally, or whether dropping the salinity does in fact have a benefit by decreasing metabolic demands. I don't have an answer to that question, I just thought I might just toss a bit more fuel on the fire.
Olin