Reading about it now apparently it's illegal almost everywhere except for some parts of Asia and is largely ignored by local police in Thailand even though illegal along with most gambling.
I have never come across anyone fighting fish, or even considering it. Lots of people keep bettas, and they are easy to find. Purely ornamental.
There used to be a cock fighting ring a Chatuchak market, right by the fish quarter, but that was out of use thirty years ago.
The local police ... they have got much more important things to deal with.