I don't know why, but I know some people think curved tanks are cruel? I forget the reasons.
This:
Something about the curved glass distorting their vision and stressing them out I think.
And it is said that it may damage their eyes. It's also important to keep in mind that some tanks (bowl like specially) have a relatively small area in contact with the air. (Effectively reducing the interchange of gases)
And just in case, tall tanks look gorgeous, but are TERRIBLE to keep: proper illumination for healthy plans becomes a nightmare, water flow becomes a royal pain (Can't be too strong and yet it should "rotate" effectively the water from the bottom to top and vice versa).