I can think of one, which may possibly apply to other pathogens as well.
http://www.veterinaryresearch.org/content/44/1/27
6.1
Preventive measures
But as you stated, different freshwater species may have a different tolerance level towards sodium chloride. What may be beneficial to one species, could be harmful to another.
Good point, although I think I was working along different lines.
There is ample evidence that salt is useful:
1) to reduce stress during fish movement and capture
2) to control or eliminate parasites
3) to encourage feeding for stressed fish
4) to encourage injury repair
5) offset nitrites in the tank
(I may have missed others which was inadvertent. And I am referring to FW fish who evolved in salt free water.)
I 100% agree that it has valuable purposes. But it's value (seems) most apparent when it's used to ward off a different more dangerous condition (stress, capture, parasites, starvation, nitrites, open wounds.) When a fish is in a tank with clean water, no parasites, is not stressed, is not being moved, has no injuries, and is feeding, the value of salt is much less supported. Preventative---curative---absolutely.
In the absence of need, I would hypothesize it's more like taking vitamins for someone who is already healthy and eating a healthy diet. Not needed, but the more you take, the more injurious it becomes.
Salt for freshwater fish may indeed be good---regardless of the presence of parasites and injuries---dosing a tank at low levels with salt may indeed have good LT benefits. Perhaps if one takes certain (or many) species and raises them from fry to fully aged adults, they will grow better and live as long (or longer) than fish not raised with salt. I'm just skeptical.
Here's a few studies I found that tried to evaluate various salinity levels effects. I wish I could say it was clear, but the effects are all across the board. Some good effects, some bad effects.
http://fl.biology.usgs.gov/projects/tilapia_salinity.html
http://www.tandfonline.com/doi/abs/...<0545:GOLBIL>2.3.CO;2?src=recsys#.U-Q0WvldVZ8
http://pubs.er.usgs.gov/publication/70056141
http://www.arkansasstripers.com/salt-and-shad.htm
http://www.thefishsite.com/articles...e-on-atlantic-salmon-postsmolts-reared-in-ras
http://www.ehp.qld.gov.au/water/pdf/potential-impacts-sal-tur.pdf