This topic comes up quite often, and many still do not grasp the issue. Not casting aspersion on anyone, I will just try to explain the reality.
Male Bettas are not community fish. They should always be on their own in a suitable tank. The reason this is fact has to do with the inherent normal and natural make-up of the fish. The DNA. Ichthyologists who study a species are able to discern its inherent behaviours. We take these to be the way the individual fish in the particular species will behave, interact, respond to external stimuli, respond to the environment, etc. It describes the species, in effect, so far as these aspects of its physiology and life are concerned. It is the "norm" for the species, and the majority of individual fish in that species will adhere to these norms.
Individual fish within a species may not always follow the norm. There are several reasons for this. Any environmental factor that the fish "expects" but which is not provided can affect its behaviourial response. For example, it is well established that when a shoaling species does not have sufficient numbers of its own, aggression will increase in most of the individual fish. This is simply the fish's way of responding to what it sees as adverse environmental conditions. Sometimes the fish's response will be the opposite. But what must be understood is that the fish has expectations programmed into its DNA and when certain of these are missing it is going to react somehow.
Back to our male Betta. Placed in with other fish, the betta's "normal" response is to drive the fish out of his territory. The fish purchased today have had this response heightened through breeding. So we should expect trouble, and the only way to avoid harm to the betta or the intended fish is to recognize the inappropriateness and not push the fish into what is for it an unnatural and foreighn environment. That is responsible fish husbandry.
It must also be understood that what we see is not always what is occurring. The fact that Betta "x" does not physically attack the other fish does not mean it is not still responding normally to an unacceptable (for the Betta) situation. The betta still regards the space as "his." He will be releasing chemical signals called pheromones (read by others of that species) and allomones (read by other species), and these can cause severe stress just as much as physical interaction, even though we cannot see them. We may however see the result, sooner or later. More than one member here has reported that all was well with their betta and his tankmates, only to have the member wake up one morning and find dead fish and the betta laying down the law.
It can work in reverse too. Small shoaling fish when confronted with a betta can also respond physically, nipping fins is common, or via allomones. All of which will stress the betta, maybe unseen, until it is too late.
The bottom line is to accept the norm for a species and provide accordingly. Anything else is taking a risk,and responsible aquarists should not deliberately go down that road. It is not fair to the fish, and it is in fact inhumane. As Paul Loiselle, an acknowledged cichlid authority, says in the citation in my signature.
