I fixed a pinhole leak on the bottom of my tractor's tank on a seam. As pointed out, if it is just a 500 gallon tank, likely rusted out, just get rid of it. What worked for me was to weld a "slug" of metal over the spot that was leaking. I had it right full of diesel. It was a bad drip, several gallons /day. On the slug, I drilled and tapped the "slug" for a 1/8"or 1/4" pipe fitting - I forget. It was a dirty messy job that I only did because I needed the tractor to finish seeding. I then welded it in place over the pinhole area and cleaned and put out fire as necessary. The leak found itself to leak through my plughole. Once I had a pressure tight seal, I turned in a pipe plug to stop the leak.
The reason behind my issue was that I blew air into the diesel tank to start the tractor in spring. I was having difficulty priming the pump as there is no device there to do that. The weakened seam developed a pinhole leak, likely covered over by paint and finally let loose. The leak didn't develop until 5 days later and then I analyzed how it began! Expanding the air with low air pressure may have contributed to it!