dragon wrote:
My bettas are all in unfiltered 2.5 gallon tanks. How often would
ya'll recommend I do a 100% water change? I've been doing it once a
week in the past, but the fishkeeper at my LFS thought that was too
often. So, what does everyone think?

Water change enough to keep your parameters (esp. Nitrate, Nitrite &
Ammonia) low and *stable*.

You also might be asking the question slightly wrong. It may be better
to reduce the amount of water changes, rather than the frequency. (e.g.
do a 20% every week, versus a 100% every month.)