This has been a really interesting thread to read through! Although I feel like the first post definitely needs to be updated, with working links to sources.
Now I feel like my 6L tank may not be enough for my fish - though for a long time he seemed to do very well in it (although I don't know very much about fighting fish behaviour and can't be sure)!
It's kind of silly, though, to compare wild and domestic counterparts of animals. Wolves (I'm not so sure about other wild dogs) and lions go through "starvation" periods in the wild where they are unable to bring down prey, and this is normal. In fact, some zoos try to emulate this by not feeding their (wild) animals some days. However, in terms of our companion dogs and cats, it is unthinkable that we would starve them one day of every seven. So, to say that for half a year a fighting fish would normally live in less than 30cm of water, therefore the same should be done in a household tank is a little ridiculous.
Just because something is done a certain way in the wild, does not mean it is the best way.