I have to also agree with the above replys, but have encountered these "pin holes" on quite a few water tanks and irrigation systems made of galvanized as well as the tanks with an inner coat of some kind of glassy material found on hot water heaters too, not to mention air compressors and other tanks. Hopefully, they are actually nothing more than pin holes and therefore not what I usually run into, which are layers of rust which are somehow held together well enough to simulate only a pin hole leak (usually these are around welds made prior to painting and tank completion). Problem is, when you take a wire wheel or brush to the spot, the rust starts flaking off rather easily, and you end up with a hole 2 inches in diameter with metal surrounding it that is about as thin as a piece of paper. Hope this isn't your case, but if it is, I know of several ways to repair successfully and just holler if you are curious.
Hope that helps!
G. L.