Of all the hundreds of thousands of rounds spread across however many blocks and just two ended up hitting the tanks. Uh, no, he wasn’t aiming at the tanks. If he’d wanted to blow them up, there would be countless holes in them. They’ll be finding random chinks in buildings from this for the next 20 years.
The tanks are quite a bit farther from the Mandalay than the concert venue, and in a different direction. Shooting at night, with no tracers, he might have been unable to get the proper range for them. There’s nothing between the Mandalay and those tanks except empty dirt lots, so there’s probably a big chewed-up patch in one of them full of bullets. The two that actually hit could have been lucky hits that he thought had gone high.
And he didn’t expend “hundreds of thousands” of rounds, spread across many blocks. Most of his fire went into the concert area. He maybe emptied one of those 90-round mags in the direction of the tanks if that.