I agree. All tetras must have a group, and those that do not will often turn aggressive. Sometimes this is rapid, sometimes more drawn out, but it is probable.
There is another thing here too, which may be relevant. The Columbian Tetra is a more feisty tetra, and one that should never be combined with slow swimming and sedate fish like all gourami. It is possible that the tetra has been after the gourami even if you were not there to see it, or this can also occur chemically; fish release pheromones and allomones that other fish "read" and aggressive signals can be just as stressful as actual physical aggression. The dwarf gourami is not all that peaceful a fish itself, depending; it may be returning the aggression, having had enough. If the tetra was in the necessary group, the collective feistiness can make it too difficult for a picked-on fish to turn, but depending upon the temperament of the gourami this may be the case here.
Whatever the explanation, these two shouldnot be together, and the Colombian Tetra if you stay with that species must have a group of six or preferably a few more. But it is a feisty and more active tetra so keep that in mind. I've no idea what other fish you have or would like, but a tetra like this one does limit options.
