I would also say a tumour of some kind. I can think of no critter either water bourne or land based that looks like that. It's definately not a larval stage of a dragon fly though. I'd make the effort to catch the fish and have a go at removing it.

BTW Susan Freshwater fish don't need any additional salt added to thier water. There's sufficient naturally occuring sodium and chloride ions for anything that our fish need. That 0.08% would have zero affect on any bacteria or parasite