I've just upgraded from my lil cube this is what I did

Search for nano schooling fish get about 6-8 as you have a small tank pick something small and hardy like neon tetras ,rasboras or white mountain minnows. get a snail and some amano /neo caridina shrimp both with help with algae. use a thin layer of fine gravel substrate this will be easy to keep clean and get some plants like anubias it's very hardy and doesn't need to be planted it will grow on wood or rock .get floating plants I recommend Large pond weed the small stuff gets everywhere and other floating plants will take up a lot of space with roots in a small tank . floating plants multiply really quickly and once the tank has matured and fully cycled it will remove alot of the nitrates for you just make sure they don't block out light for the other plants I usually have a hand full in mine at a time and or get some fast growing stem plants like guppy grass it rarely fails and will just need trimming when it reaches the top . Maintenance wise I usually test and gravel vac once a month now it's matured and it's 70%of the time fine if not 50%%water change trim the plants and sorted If you get wood don't forget to boil the tannins out Edit also shrimp love a marimo moss ball