Brackish bichirs?
Bichirs are freshwater fish and, though some are found in rivers near where they empty into the ocean, the water they inhabit is fully fresh.
Polypterus ornatipinnis and Polypterus e.congicus are sometimes found in Lake Tanganyika which has hard, alkaline water but are rarely, if ever, found in the lake itself. Instead, they inhabit the swampy areas where river water empties into the lake proper.
Yep, I hit up fishbase too!