I had the same problem with my cube. I think tall aquariums in general aren't good because there's more vertical than horizontal swimming which lead to ultimately less space & more fish encounters. I had to beautiful & thriving tangs. One was a hippie tang & the other a yellow, both babies, & were super confident about her environment. They began fighting every time they ran into each other which didn't happen in the small 10 gallon quarantine I had them in for two months but since the cube is more vertical, they began bumping into each other & using their attacks on each other. Didn't end well, thew yellow tang got hurt badly by the hippie tang & ended up passing away. Anyway, I think vertical tanks are just a bad idea.
For that reason, I say go with peaceful fish like the ocelaris clown fish & various anemones.