Get some weld on #16. Apply a bead along the seam on the inside of the tank. Let it set. If you can tip the tank up at a steep angle, then you can just let it pour from the tube and run down the seam. No need really to squeeze the tube, it will just run right out by itself pretty much. You might do this a couple time to be sure you get it sealed up right. Also you could try #3. It's water thin, you can get a syringe specially made to apply it and apply it along the seam prior to the #16 It will find it's way into the tiniest cracks. But you'll need to apply some pressure to do any fusing.