I'd only do a maximum of 4 discus if you wish to increase from the two as otherwise if you want to add any other fish you will need to upgrade the filter. I have cardinals, rummynose, hatchets,sterbais and fancy plecs in my tank and they look good. Other fish that cope with the temps are bn & harlequins. You could add some driftwood and contrasting plants. I currently have red pine, indian? fern, swords and dwarf sag, all of which are doing well without great lighting, with jbl balls added and flourish excel to keep the algae under check. Val looks good as well but doesn't like a lot of excel added.