Pacu Mom, every time I see that picture my mouth drops open from amazement.
This is pure art, stunning in every sense.
Personally, I'd not go with anything in the tank, only use a single color spray-painted back and sides. As for color, 2 choices, a contrasting color with your Pacu's or the same light (white/off-white or maybe very light grey) color as your room.
With this very light and perfectly crafted room, I'd want only the two Pacu's to pop out and get attention. They are the show pieces, like a painting. You don't put a frame around a painting that distracts from the painting itself. You want to the frame to be a bridge with its surroundings, the wall it is hanging on, blending it with the wall and at the same time causing the painting to pop out to the foreground.
Your Pacu's are the painting, the rest is the frame holding it.
Good luck and look forward to see the end result.
Cheers,
Luc