Someone mentioned, in a previous post, that most of the fish they "collected" from the wild were in streams that were only a foot wide and not more than a foot deep. I'd like to point out that a body of water that is a foot deep and a foot wide will have a cubic foot of water in each linear foot of the stream. A cubic foot of water is 7.48 gallons of water. A cubic foot of moving water, even if it is moving slowly, is a LOT different than a gallon, or even worse a 1/2 gallon of still water. If the stream stayed a foot wide, a linear foot of the stream would have to be no more than 1.6" deep to have as little as 1 gallon of water in it. Even if it got that shallow on occasion, a stream is still moving. A pond or other closed body of water still gets refreshed by rain, and the natural plants, mud, etc., would effectively filter and convert wastes. None of that happens in a gallon of still water, unless we do it.
In other words, the personal experience of someone "collecting" wild fish in Asia does not, in any way, mean that bettas can thrive in "tiny mud puddles."