Mirrored perspex

[sphelps]

The sides will appear mirrored when the tank if full and viewed from the front, using anything on the outside like paint will not have an effect. You could put plexi on the inside, black is a popular choice, not sure if mirrored plexi has metals in it so probably not suitable for submersing. I have used mirrored window tint on the outside of a tank before but again only useful on the back, not sides if front only viewing.