That's because it's dark in the deep.
No. The fish in the deep sea vary in color. Some even have their own lights on their body.
Black
Theragon fish goes up to 1,500 to 4,500 feet below the surface.
The black swallower, a deep-sea fish known for its ability to consume prey larger than itself, is not dangerous to humans because it resides in the deep ocean at depths of up to 3,000 meters. Its habitat is far removed from where humans typically swim or interact with marine life. Additionally, the black swallower lacks the physical attributes or behavior to pose a threat to humans, as it primarily feeds on smaller fish and invertebrates in its deep-sea environment.
The Pacific black dragon, a deep-sea fish found in the North Pacific Ocean, primarily feeds on smaller fish and various types of zooplankton. Its diet includes lanternfish and other small prey that inhabit its dark, deep-sea environment. The fish uses its large mouth and long teeth to capture and consume its prey effectively.
When tiger fish get sick, they may change color to appear darker or faded. This change in color can indicate stress or illness in these fish.
There is no known creature called a Black Sea Dragon. It is possible that you may be thinking of the Black Dragonfish, which is a deep-sea fish known for its black coloration and bioluminescent abilities.
At the current time they appear not to have been bred in captivity. No obvious differences are visible between male and female Black African Knife fish (Xenomystus nigri).
Flashlight fish typically have a dark, bluish-black body that helps them blend into their surroundings in deep-sea environments. They possess bioluminescent organs under their eyes that emit a bluish-green light, which can appear as a glowing spot in the dark. This unique adaptation helps them communicate and attract prey in the depths of the ocean.
Reef fish have bright colors to hide in the corals better. I don't know much about deep sea fish being violet, more like black, brown or translucent to hide in the darkness. Sometimes they're glowy to attract prey in the darkness.
The black swallower is a deep-sea fish that gets its name from its dark color and its ability to swallow fish lager than itself. While not too much is known about this fish, it is preyed upon by larger fish, such as sharks, and squid.
deep deep deep underwater