I have noticed that my Betta regularly flares at his reflection in a couple of spots in his tank.
He patrols quite assiduously and every time he swims past these areas he has brief showdown with the handsome devil in the glass.
I know that this in and of itself isn't harmful, but will prolonged exposure to his reflection cause him undue stress?
Would this "intruder" contribute to a lack of bubblenests on his part?
Nope, not at me, just at the fish in the mirror.
He'll chase my finger or other object around the tank and he comes to the front of his tank when I walk into the room or move once I'm in it, but no flaring.
When I first got him I never saw him flare at all, then I caught him doing it once when the room lights were out and his tank light was on, now I see him do it quite regularly.
If he's doing it all the time it can be too much stress on him. If it's only every once in a while, he'll be fine.
As you said, he will be able to see his reflection when his light is on and the room lights are off. You can combat this by putting plants or something against that wall so he can't see his reflection, or trying to keep both sets of lights on or off at the same times.
I know I probably don't have much room to speak as this is my first betta, however, I think he will probably be fine. I don't think Rosso sees his reflection in any part of his tank but the lighting in his tank and in my study, enables him to see me and he will flare at me for a little bit and go about his business.
He finds other reasons to flare as well. I honestly think sometimes he's just strutting his stuff a little. Look at me, I'm a big, bad, red betta. He had a pretty healthy bubble nest going, too. I had to clean his tank the other day, so he's probably rebuilding.
I was actually asking myself the same question, and I did a bit of research about it. My betta kept flaring at himself in his reflection at the back of the tank (and I have a background there) so I moved his tank a little, because I thought initially that the reflection from the window was causing him to see his reflection more. I also moved the plants around a bit to try and block his reflection. He still flares at himself, mostly when I've turned the light off, and when it's daytime. He does it at night sometimes too, but not as much. Maybe because he's a lil' sleepy. He has also been creating a few bubble nests and blowing bubbles more ever since I moved his tank mates.
I think he's a bit frightened of me, so rather than flare at me, he just backs away xD
I actually think it can be quite healthy for them too, so long as they don't do it all the time. So yeah, I'd say it's nothing to worry about.