Actually blacklabel, it states this is a 5 gallon bowl which is plenty big. You shouldn't keep bettas in tiny bowls or vases. And if a pwc is done every few days due to no filtration, there is nothing to be concerned with.
From blacklabel3 on 01/23/10
Shouldn't keep fish in bowls, it's not healthy for them.
GreenPlanet's 7 gallon freshwater fish tank, Sub Tank