My black skirt tetra's fins go clear when the lights are on!?

I thought leaving the light on for 10-12 hours a day was recomended, but my black skirt tetra's fins go clear whenever I turn it on, and they like to hide under the rock. There are 2 of them in the tank and the only other fish is a placo. (algae eater). They turn a beautiful black when the light is off and swim all over the tank... I've got a decent amount of natural light coming into the room all day, so it's never pitch black until night. Should I continue leaving the light off and let the natural light be enough or should I keep it on the recomended 10-12 hours during the day?