I can't offer suggestions on the behaviour because I simply don't know. But I am going to use this case to illustrate a point I frequently make, as it should help you and others avoid similar issues in the future.
All characins, most cyprinids, and some other fish are what we term shoaling or sometimes schooling fish. They must be in groups, and the number in the group impacts on their behaviour. Recent scientific studies, the first in this area, that I posted back when I came across it have proven that shoaling fish in groups under five will almost always show increased aggression. And fish species that would normally be deemed peaceful--such as neons, glowlights, etc--can suddenly become aggressive. The reason for this is clearly the lack of sufficient fish in the group. A very similar thing occurs in too small a space for the particular fish--and remember, too small here means to the fish, not to our perceptions of space being adequate for water quality and such.
The above issues stress the fish, to varying degrees. And just as with dogs, or even humans, each of us responds a bit differently to levels of stress, so too with fish. But whatever the level, it is a sign of frustration, and the fish is lashing out the only way it can. Sometimes it goes opposite, and instead of becoming aggressive it can become very withdrawn, even as far as to refuse to eat and simply waste away. In both cases it is a reaction to a situation that stresses the fish and it can't cope.
Back to the numbers; I mentioned five for the study, but there is ample evidence from the experiences of most of us that the more the better. The more fish in the group, the less the chance of stress related to the numbers, and the less chance of some reaction. The fish are programmed by evolution to live in groups of hundreds, and they have a number of needs within this group. There can be safety in numbers, social interaction, even normal aggressive behaviour (more obvious perhaps in fish like cichlids such as angels and discus). But this need is in the fish's genes.
I agree with Exterrestrial that a 10g is insufficient space for two species of shoaling fish. A 20g long would be much better, if that is possible. Or just one species, with 7 of whichever. These are the ideals. But another point has to be made, and that is that the effect of stress-related issues doesn't always show itself soon, but can linger and may or may not show up later. Stress weakens the immune system too, so that is another issue. And stressed fish always (as far as I know) have a shorter than normal lifespan due to the weakening effect of stress.
Hope this is of some help in understanding the cause.
Last edited by Byron; 03-23-2012 at 01:12 PM..