I've found the problem. It seems you've tried to create a cylinder with 8 faces, on a small brush with measures about 4 units, by 4 units.. Since the brush was very small, it was unable to create a cylinder causing trouble. As you can see in the snapshot it appears to have more faces. (2 faces per side.) This accident can happen very easy, especially, when you use the cylinder tool, then go to make a sqaure brush, but for get to change the option back.
By the way, I'm extremely impressed with the tank, it's unreal!
So ya try that, and tell me if that solved your problem!