As Moses and the children of Israel are crossing the Red Sea, the children of Israel begin to complain that they are very thirsty after walking so far. They can’t even drink from the walls of water on either side of them because they’re made up of salt water.
While Moses is looking around for some fresh water, a fish from the wall of water tells him that he and his friends are willing to help. They will use their gills to remove the salt from the water and force it out of their mouths like a freshwater fountain for the Israelites to drink from as they walk by.
Moses accepts this kindly fish’s offer with gratitude, but the fish says there’s a condition. The children of Israel and their descendants always have to be present at the Seder meal that will be established to commemorate the Exodus, since they have a part in the story.
When Moses agrees to this, he gives the fish their name, which remains how they are known to this very day, for he says to them, “Go Filter Fish!”
From Oy Vey: More! The Ultimate Book Of Jewish Jokes Part 2, by David Minkoff