Maybe he cycled his tank with tetras back then, a lot of people that will keep aggresive/predatory fish in a tank cycled by tetras, danios or whatever knowing they'll get eaten later on ... *shrug* I dont think its that bad honestly
Tennis Balls and PingPong balls are used for oscars, they'll push them around and "play" with them to stop the from getting bored when they're in a tank on their own. I guess thats why this ball is in there too ... far as I'm aware, its done quite often and not a risk for the fish. The bottle I dont know, though I doubt its any worse than keeping lil castles or anything in your tank, these too can break.
I agree, actually ... I believe the fish DID change, why this got brought up again, I dont know.