There is some debate about whether or not hexagon fish tanks are “fish-friendly.” This may seem like a silly question. It may seem that fish are happy as long as they are fed and remain in a clean, well-maintained environment. This is true to some extent, but don’t you think that fish would be happier swimming about in an ocean or sea or more “natural” environment? Yes.
And it is in this sense that they may not be as satisfied living in hexagon fish tanks. While no specific studies to note have been done to ascertain whether or not fish are more likely to become ill or have lessened lives while living in a hexagon-shaped tank rather than a long rectangular one, there are reasons to believe that lifespan could be decreased by such an environment.
Fish can swim better when going back and forth in long, rectangular tanks. In hexagon fish tanks, on the other hand, fish can only swim around and around. The motion is less natural.
That being said, hexagon fish tanks can be very attractive and interesting additions to a home. If you really want one, make sure that you get a large enough one with lots of swimming space. And make sure that you buy small fish.