Depends on the color of the fish and substrate.
Darker or deep backgrounds are best for metallic or bold colored fish.
Paler fish look better with paler backgrounds, gives an iridescent effect and shows subtle color. Dark deep backgrounds overpower pale fish, making them washed out.
Or you can go complementary color to make the fish POP like a neon sign.
White fish - blue, black, green, brown, or red background
Black fish - pale blue, pale green background, gray background, dk red background
Yellow fish - bold blue background, blue-violet background
Red fish - green background, blue-green background, black background
Orange fish - blue-green background, blue-violet background, black background
Green fish - red/burgundy background, black background
I've been in pet stores with different color backgrounds to enhance the appearance of the fish for sale.
I also used to crush craft paper and spray paint in shades, or crumple aluminum foil on the dull side and tape to the back.