Pencilfish night time colours

Bought a group of these (nannostomus trifasciatus) on Saturday and been going to the tank early every morning to try to observe this. Assuming it was the stress of being moved that surpressed this but they had finally changed this morning.

Pics are not my fish
Daytime

Night time

All known species in Nannostomus with the exception of N. espei have one or more horizontal lines in the pattern. During darkness, these horizontal lines break up into a series of dashes such as you observed, and is shown in the second photo in your post. The only exception is N. espei which has this dash pattern permanently (photo attached).

This has been observed in blind fish, showing that it is an automatic response to light/dark and not controlled by the fish. This is another example of how light impacts fish, even when blind; the cells are light sensitive and the circadian rhythm adjusts accordingly.

Nocturnal colours in fish are normal and nothing to worry about. However, stress from tank lights coming on when the room is dark can be an issue. Fish don't have eyelids and don't tolerate going from complete dark to bright light (or vice versa) instantly.

In the morning open the curtains or turn the room light on 30 minutes or more before turning the tank light on. This will reduce the stress on the fish and they won't go from a dark tank to a bright tank instantly.

At night turn the room light on and then turn the tank light off. Wait 30 minutes or more before turning the room light out. This allows the fish to settle down for the night instead of going from a brightly lit tank to complete darkness instantly.

Colin_T said:

Nocturnal colours in fish are normal and nothing to worry about.

Click to expand...

Not at all worried. As I said in post #1 I was expecting this and looking out for it. I've never had fish before that changed so dramatically and thought it was cool that they did exactly what the pics and articles said.