I have heard that bettas require a little more effort than goldfish. How big is your tank, and how often do you have to clean it out?
We have a 2-gallon tank with a filter, but we kept Howard in a vase, and he did just fine (except when Agnes drank most of the water while we were on vacation!). We cleaned the vase about once a week, just put Howard in a water glass for a few minutes :-). If you overfeed bettas, their water needs to be changed more often. Otherwise, they're very clean.