The ornamental form of Betta splendens, remains the most aggressive species of the genus Betta. While originally bred to fight, nowadays, Betta splendens are celebrated more for their beauty than for their brawn. However, this has done little to dampen the species' aggressive streak.
Aggression comes naturally to bettas. I have witnessed fry only a few millimetres in size sparring with each other over food. Often, fighting between fry and subadults is restricted to intimidation and display. However, sometimes physical damage can be inflicted. It is perfectly normal for fighting to intensify and become more serious as the fish near maturity, particularly in male dominant spawns.
I have found that with wild bettas, aggression tends to vary wildly between individuals. An example is the difference in aggression levels between my captive-bred and wild-caught Betta burdigala pairs.
While I rarely see any damage to the fins or scales on my wild-caught pair, my captive-bred pair can be extremely violent. The level of aggression peaks during courtship and occasionally when the male is guarding the nest. The damage shown below is very minor compared to what has been done in the past.
I think sometimes those new to this side of the betta hobby, mistakenly believe that wild species are completely nonviolent. From what I have experienced in the past couple of years, I would say that it is some of the smallest species of betta that can be the most aggressive. For example, the strain of rutilans I keep, can be particularly nasty. I once had a juvenile attacked and killed by its siblings and parents, and even in the grow-out tank they fight incessantly.
With that said, the majority of fights between my wild species, seem to be fairly short-lived. Usually the fight ends with a quick chase around the tank until the pursuer becomes distracted or loses interest. Quite often the whole 'fight' is just one giant chase with fish constantly jumping in and out of the action.
It's always good to provide some cover and hiding places for your wilds, particularly if you have individuals of various sexes living together. Even a thin layer of leaf litter can provide a place of shelter for a harassed fish.
Perhaps the most important thing to remember is that bettas are all individuals. While one pair may cohabitate peacefully, another pair of the same species may routinely tear strips of each other. Nothing is ever certain with these fish, particularly when it comes to sharing tanks!
Hailing from Melbourne, Australia, I currently keep and breed a number of species from the coccina complex.