I haven't heard anything about hexagonal tanks driving fish crazy, but my guess would be that, if used as a centerpiece, they could easily stress any animal, as they would be fully exposed to regular movement of human activity from all sides. Of course, the design makes it tempting to use these tanks as centerpieces, but I've always put them against a wall.
I have my big eyed treefrogs in a 20G Hex, and they've been fine. I do cover the rear half of the tank (I used paper at first, and eventually upgraded to a matte black spraypaint on the exterior). I do this, partly to help the frogs feel safer and less exposed, but also because the tank is in an area with natural light, so I wanted to block the rear three walls (most exposed to sunlight) and prevent drastic temp/humidity fluctuations.
I have used hexagonal tanks as fishtanks. The problem I've run into: they tend to be made of plexiglass (easier and cheaper to mold into a hexagon) rather than glass. They work like a charm... unless/until you run into algae problems. As soon as you try to get algae off of plexiglass, your tank is likely to become useless. Anything strong enough to scrape algae off effectively (other than a Pleco, perhaps) is strong enough to scratch plexiglass, and it will loose transparency. I ran through 2 hexagonal, plexiglass tanks before learning that lesson. (They are now used as ornate planters). I won't use anything other than glass for an aquarium. That said, my treefrogs have done just fine in a hexagonal, plexiglass terrarium.
