What kind of light do you have? LED, flourescent, color temperature, wattage, etc? That will help.
Floating plants typically have low light requirements because they're pretty much directly under the light anyway. Frogbit, salvinia, water sprite, etc are common choices. All help greatly with water quality, which will be useful for you having a large fish in a relatively small tank. It's generally recommended to stay away from duckweed because it grows really fast, makes a mess whenever you have to go into the tank/during regular maintenance, and is almost impossible to get rid of when hyou get tired of it.
As far as other plants go, Java Fern and Anubias are usually recommended as hardy low light plants. They are attached to rock or driftwood instead of planting, and they have minimal light requirements. I've personally found Vallisneria to be hardy and do well with moderate lighting, makes a nice grassy background wall of the tank. Some Crypts are okay with low to moderate light as well. Java moss is pretty if you attach it to driftwood or rocks.