i always change my betta bowl every week, but this aquarium has a filter so i might start changeing it every every week and a half.
i dont think that puting two bettas together will stress them, i put a mirror up to my betta's bowl, and all he dose is flares at it once and swims away, it dosent seem to bother him.
here is a pic of the tank
http://www.walmart.com/catalog/detai...uct_id=3635102