I think it looks chaotic because the background is light collored there is no contrast. If you painted the background black it would make contrast from the background and plants. I know my planted tank always looks empty because of the background so it would probably look really good on your tank. Since your tank is already filled try the tape on stuff to see how you like it. I still prefer paint as there are no imperfections present but that means taking the fish out and stuff so thats a big chore.
no doubt its a big chore haha, i just put my black neons in yesterday and it took me prob an 45 minutes to catch em all and get them moved, not to mention the little keyholes which are surprisingly fast when they get spooked. no way i'd do that again soon if i can help it. i have some black poster board i've been meaning to tape on the back but it's awkward and i'm lazy, so it just has been sitting on the back burner