Neither GMM nor Plano do have Circular Stairs that Richie was talking about. The type you are talking about [on a tank] is a totally different animal. Circular Stairs around a single post is quite easy. Going around a tank diameter is a different story. The problem is that the stair treads start to get in the way of each other because of the diameters of the inner panel and the outer panel.
It can be done, I have done it. The caveat is that the stairs must be made exactly for the tank diameter you want to put them on. They can not be made for "general" use. I have wrapped one of my 7.5" stairs around a tank to try it. It did work, but looked real funky so I used shorter pieces and put landings between them.
