You could try a dwarf angelfish species like a Rock Beauty, Coral Beauty, Flame Angel, Cherub Angel, and others like them ive had some of them species in a 26 gallon bowfront before and all did fine. Just take caution doing an angel in a tank that size if you plan on stocking corals in the tank because the chance of them eating your corals are super high in a tank like that. Royal Gramma is another good one. I would look into dart fish like the red, purple, and helfrichi dart fish they are so awesome make sure you have a lid with no space even around overflow because they will find a way to jump out trust me it happend to me a lot. Diamond watchman would be a sweet addition to the tank and they are a utilitarian fish that will clean your sand bed for you by sifting through it and eating waste detritus and leftover food. A blenny is also a must something like a tailspot blenny would be sweet as well and they are also a utilitarian fish because they will graze on algae all day. If i was to suggest this is kinda what my stocking list would be
2 Red Dartfish
1 purple Dartfish
1 Helfrichi Dartfish
1 Flame Angel
1 Tailspot Blenny
1 Diamond Watchman Goby
1 pearly Jawfish
1 Royal Gramma
1 Solar Fairy Wrasse
That would be the perfect stocking plan for a 30 gallon cube good amount of movement a lot of different colors and there is still a lot of swimming space because there is only two mid to top water swimming fish all the rest hang out bottom half.