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If the tank is bone dry and absolutely clean, you probably can seal the crack with MarineTex, a high quality epoxy material. I'd overlap the crack by applying plastic (or copper, if you can still find it) window screen material for about 2" on either side, and then press the marine-tex through that, forming a mesh reinforced patch with plenty of surface area.
I'm not sure if MarineTex is recommended or approved for use in drinking water tanks, but if not, you should be able to find an epoxy that is.
If you can't fully access the crack, the other conventional alternative is to put a blivet (Nauta expandable bladder, etc.) inside the tank as a liner. It will fill the entire space once the baffles are removed, and if you place another material in between the blivet and the tank (like rubber matting) to prevent chafe, be very reliable.