Ross Vandegrift wrote:

I suspect the original poster wasn't
completely joking --- a filterless tank will need water changes
frequently. If I were going to do it, I'd shoot for daily changes.
Lots of people do fewer if they keep only a single male betta, and
they do ok, but I've rarely seen them do well. I've seen plenty of
them dying from terrible dropsy and bacterial infections.

On the other hand, in a small enough tank those water changes can be
accomplished with a gallon water jug, a small bucket (or second jug)
and a turkey baster, in about five minutes. That's how I maintained my
original "I'm just going to get one fish, honey, honest" betta tank.

--
Eric Schreiber
www.ericschreiber.com