Bowls specifically are worse than large, filtered, rectangular aquariums because they're typically too small (causing stress), unfiltered (resulting in waste buildup which causes stress), round (which can act as a lens if the bowl is small, which provides an unnatural, constantly-changing view which causes stress, and which also offers no corner to hide in or rest), and open at the top (so things can fall in, fish can leap out, and cats can attempt to catch them).