Rav Yaakov Yitzchok, the Chozeh had the custom to salt the fish himself in honor of Shabbos. He did so while still wrapped in his tallis, crowned with his tefillin and meditating on special yichudim – lofty unifications of the Divine attributes after Shacharis. We ourselves witnessed this as well as heard from others’ holy mouths as well.
He once invited Rav Hirschel of Zidatshuv to come and taught him the secret of salting fish in honor of Shabbos Kodesh (Rav Yitzchok Isaac of Komarno’s testimony in Shulchan HaTahor 250:3 and Neziv Mitzvosecha Emunah 6:5)
When Rav Yaakov Yitzchok arrived in Mezritch for Shabbos to the Maggid’s table, Rav Shneur Zalman of Liadi, the author of the Tanya and Shulchan Aruch was there. The Baal HaTanya saw him enter the kitchen and say to the cooks, “I always have the custom to salt my own piece of fish that I will eat in honor of Shabbos.”
Rav Shneur Zalman was observing this strange behavior wondering what would happen on Shabbos since this piece would get mixed up and cooked with all the others. So, he took a piece of string and tied it around the fish to mark the fish that the Chozeh had salted.
On Shabbos as the talmidim sat around the table and the fish was distributed, Rav Shneur Zalman watched and waited. Sure enough, the fish with the string tied around it ended up in the plate of the Chozeh’s neighbor and not on his own plate. However, the man lifted his fork to eat it, but mysteriously lost his appetite.
“Here,” the neighbor said to the Chozeh, “you eat this fish because I cannot eat it.” He then handed the Chozeh his own salted shtickel fish! “That is when I knew I was in the presence of someone great indeed!” declared the Baal HaTanya. (Niflaos HaRebbe 186)
