Well, The salt and potash and all sound authentic, but just slowly simmering the skins or air-bladders until everything falls apart should work, too. I haven't made fish glue yet, but hide, or sinew glue take nothing but water, low heat ( ~ 150 degrees F) and time. After 24 hours or so there's not much left of hide or sinew. Just strain and cool. If there's still fine debris it will usually settle to the bottom before the glue gels and you can cut off the dirty layer. Reheat the strained glue to use, continue slow heat to thicken, or add water to thin. You can refridgerate for a few days to store, or for longer storage you can freeze it. Ron