Before treating with prazi, do a FW dip. If it is flukes, some of them should fall off and into the FW container. This is not a treatment, you will still need to use prazi, but it will confirm for you whether prazi is the correct treatment.
Further: prazi is not entirely reef safe, it can kill feather dusters and ornamental worms, plus a lot of beneficial hitchhikers on your live rock. Treating in a hospital tank would be your best bet.