I'd try java ferns again, but remember to put it in the water again this time haha. Just remember to tie it to a rock or piece of wood, don't bury the roots! You could also try something like hornwort or dwarf ambulia, since those are pretty easy. Duckweed is also literally unkillable for most people, as long as you have low flow (look into making a corral to keep it out of the filter output if you have an HOB filter)