Hi..

Wouldn't call a 2g puddle a tank..! ;-)

In my mind the absolute minimum tank for a male fighting
fish were a well planted 10g tank but would really try to
give him a crowded_planted 20g tank..!

Fighting fish are "lakers", means they prefer the (crowded)
planted shallow water zone. In such an environment he might
possibly show some of his natural behaviour..!

Don't know any filter *slow* enough for filtering a 2g tank.
Attention: Betta is no moving water fish..

--
cu
Marco