As a fellow cube aquarium owner (mine is smaller than yours), they are - imho - the worst aquarium design ever purely due to the lack of space for fish to do what they do naturally.....swim.
Depth and width of an aquarium can be adapted to by a fish...but length or the lack thereof is an absolute 100% headache cos 99% of fish want length to swim along to thrive and a cube of 20 gallons or less simply doesn't deliver the space to stretch their proverbial fins and get some speed up before they have to start again in the opposite direction
Going round in circles is not what fish do well, they can get any speed up doing that.....think of it like a bear in a 20ft x 20ft cage going slightly insane with the lack of not just space to wander but lack of space to just chill out and get away from that damned human looking through their front window all day.
Shrimp, a Betta, snails...maybe Minnows...but very little will thrive in a small cube...live in one, yes, thrive not so much.
Mine has Fred, a very opinionated Plakat who is currently beating up some newly placed mini moss balls and showing them exactly who is boss.