for a background... acrylic paint works best for me. I have both of my tank painted black. The paint will stick with no problem to clean glass and gives you a flawless flat background. you can paint whatever color you'd like, but most go for blue or black (IMO black makes the fish pop and stand out more)
The balas: idealy wish to be in school of 6 or more for safety and are very active and skiddish fish. your 400l may not be big enough for their activity level and for their full grown size (10 to 14")
The sd's are will also get big too, the size of small dinner plates (6 to 8" around in some cases) and they will need lots of room as well.
Plus your o needs a 75 to 90G tank by itself so including the other fish, things may get tight. would recommend at the very least to look into similar sized tank that I have, 125G (~500liter), and get a wide tank over a tall if possible as this will give more lateral freedom to your fish. None of them need a high tank
One last thing, and this may seem to want to make the tank look bare, but i believe it is for your fishes safety. the jagged rock work you have in there is not the best choice for skiddish fish like balas and clumbsy fish such as o's. Both can either get spooked or just not pay attention to where they are going and seriously lacerate a fin or their scales which could end up killing them.Just my feeling that having rough rock such you have may not be the best choice with these species of fish
