I must add my agreement to what everyone is saying about this mix of fish. And perhaps I may be able to add some insight that will help you and others reading to better understand the real problem.
The confines of an aquarium, regardless of the size, creates an artificial environment to the fish we place in it. We put species of fish together and in many if not most cases the fish would never see each other naturally. And before it is suggested that tank-raised fish don't have these inherent traits, they do. We cannot re-programme fish behaviour or physiology. This is why it is so important for an aquarist to understand the habitat and requirements of every fish species he or she acquires.
When non-compatible fish (and there are many reasons besides aggression that make fish non-compatible) are forced to co-habit this limited space, one of two things usually occurs. Sometimes they can become more aggressive; this is their only way to lash out from their frustration at being placed in a situation they do not expect and in which they cannot properly function physiologically. But sometimes the opposite occurs; fish withdraw because of the same stress and frustration, and their behaviours are not "normal," showing that something is seriously wrong. This reaction usually causes health problems down the road that would otherwise never occur, and almost always a shorter than normal lifespan.
Practical Fishkeeping reported a study a year or so back that was the first to look at numbers of shoaling fish, and that study found that when a shoaling species (shoaling means the fish naturally live in groups, usually numbering in the hundreds) was kept with less than five fish, increased aggression was observed. Naturally-aggressive species became much more so, and peaceful species became aggressive. The study concluded that this is a response to a highly-stressful situation for the fish. I mention this here because it is further proof that fish have definite needs and requirements that vary from species to species, and if we want the fish in our aquarium to be healthy and "happy," then we must reasonably provide for those needs and requirements.
The citation from Dr. Loiselle in my signature sums it up. And Nathan Hill's quote is apropos as well.
Byron.
