Oh right, I have a Chi but they are not mirrored. However I did had a black backing on it when I first set it up and he spent all his time flaring up at himself, as soon as I removed it he settled down.
I am fairly new to Betta's so I am not sure if they get stressed by staring at themselves, I am sure someone more experienced will be along soon
I have to be honest, I have some dark stones, like little mirrors, that every once in a while my fishy's do notice themselves in, and they start having an arguement with themselves, but for the most part Honestly..they actually get used to it, they look every now and again, but don't stare, How long have you had the tank, if it's new~then it maybe just the fishy getting used to seeing himself, after he sees himself more often, he may be ok, unless you see a lot of flaring, just staring is not bad.
To Aus. I had a little tiny shark named Sid Vicious! He died as well.. I have one plant up against the mirror but he just goes around it. I'm watching my fish right now and he doesn't seem angry or anything he's just straight up staring at himself and keeping still.
To Lelei. I haven't see him flare. And it's a brand new tank. If he starts flaring I can move him into the 10 gallon tank I have.
I just put a lil mirror up for my lil fella yesterday, he flared a tiney bit, but what cracked me up is that he kept rubbing up against the tank at himself, like a cat rubs his body against someones leg.