It's really late here so I have to get to bed, but I'll talk your ear off about it tomorrow, promise
Most plants seem to like hard water, mine is pretty hard at 253ppm, and the plants flourish. Avoid liquid CO2, it's not really liquid CO2, and can be harmful, and definitely isn't needed. Ferts usually help though.
Anubius, java fern, any of the mosses like java, christmas, weeping or spiky moss do well in low light conditions, and cryptocorynes are also a great low-tech plant that will manage in most any tank, with a huge variety of species available.
Firstly though, I'd recommend a floating plant of some sort! Fish really like the cover it provides, makes them feel safer, open water is dangerous to fish, 'cos they don't know that there are no predators in our tanks
Also great for sucking up ammonia and nitrates. I'd avoid duckweed, 'cos that stuff is a nightmare, but frogbit (in my photo) water lettuce, salvinia or red root floaters are all good.
Lots of other great plants out there too, have a browse and see what appeals! Look for beginner friendly plants, and most lists will be plants that you can grow in most tanks with some root tabs and ferts, no special lights or CO2 needed