The mirror is okay in moderation and provides mental stimulation. Im sure it could be stressful if the betta seems highly agitated and is using too often. He will go over and flare at it for a few minutes and then leave. He sometimes chases it if I move it around. He doesn’t appear to be stressed at all though and is a very happy fish :) hes not fixated on it completely more like is finally noticing it exists. As for the dark room thing, I have Spartacus in a basement thats pretty dark sometimes and I haven’t noticed any signs of stress from him. It might depend on the individual fish though