tiny leak in black iron fuel tank
I recently had the same experience. The leak usually signifies rust perforation, usually in the lowest portions of the tank. That is where moisture in the fuel will settle. Sometimes the rust may come from the outside, but not the norm.
There are linings/coatings that can be installed, and possibly a welded patch. But the costs of these two fixes approached or exceeded the cost of a new tank. I ended up replacing with black plastic tank for around $175.