There seems to be little visual information on tanning fish skin available, at least online. This is my attempt at tanning salmon skins (store-bought). Through piecing together bits of written information I have found here and there.
These skins have been soaking household ammonia for about 5 days...traditionally it was done in urine for about 2-weeks....Then THOROUGHLY washed! Urine over time breaks down into ammonia, which then breaks down any fat, oils, and flesh left on the skin.
Next, the skins are laid out to dry. They were laid on a damp towel, and tallow or rendered beef-fat rubbed into both sides. Then rolled up in the towel and wrung out really tightly. After realesing from the towel, they were allowed to dry a bit, then had to be worked a bit, but then they became very soft and supple.
*Excerpt from Article: Fish skin artists share their skills at Smithsonian Center "...I wanted to do a garment," Nielsen said. "A parka or mukluks. Fish skins were our rain-gear."
Nielsen works with trout, Dolly Varden char and salmon -- even spawned-out salmon, whose brick-red skin can remain durable even when the meat is no longer fit to eat. She said she prepares the skins the same way she would a moose hide. "I just dry and scrape the skins with a knife," she said. Then she applies oil or other chemicals to keep the skin pliable..."