I've tried placing cardboard next to the glass, I've also tried turning a lamp on because the natural light seems to cause more reflection. Is there anything else I can try, or that you have tried that works?
I'm guessing you want to stop reflection to stop your betta flaring at himself if this is true as i understand it some flaring is good for him but him doing it all the time can be bad. I might be wrong but i read it somewhere 1 of the forum's more knowledgeable betta keepers will correct me if i'm wrong
I wouldn't overly worry about it. He'll chill out after a while once it sinks in that he cant get to the mystery betta that looks like him