When To Use Black Light

[Guyver]

Hopefully my query here doesn't sound to silly, just asking in case I'm doing something wrong.
I bought a aquarium light not long ago and it has 3 bulbs in it, 2 regular florescent bulbs and one black light. There's two switches on the unit allowing you to switch either one of the light types on and off individually or simultaneously.
My question is which light is the best to give the fish a preferred environment, or should I switch between each light to mix it up?
The fish that are in the tank are mollys, black neon, black widows, various tetras, red barbs, clown loach and various catfish.

Thanks.