scats and monos... but they're brackish. puffers, african cichlids, pim cats, rays, neon tetras, gold tetras, silver dollars and prehistoric monster fish imo.
to me, fish with very vibrant and/or metallic colors look like what i would generalize saltwater fish to look like. also, fish that are closely related to saltwater fish (puffers, pmf, rays, etc...)
almost forgot glo-fish, bettas and a bunch of the killifish
bull sharks live just fine in fresh water they have even been suspected of breeding in fresh water in rivers in austrailia and sharks from the genus glyphis are thought to be true fresh water sharks