Unfortunately this was inevitable. Tetras are shoaling/schooling fish, and that means they absolutely must have a group. Six is/was the often-cited number, but we now know that is not sufficient. I would aim for 10 of any of these species. In smaller groups, the fish show increased aggression; normally aggressive species become much more so, and normally peaceful species become aggressive. It is usually to others of their own initially, but it will spread to any other fish in the tank that thy take a dislike too. I've seen it in my own tanks when through old age a shoaling species is down to three or four. But we have scientific studies proving it.
In your situation, the Columbian was alone, and eventually this took its toll. And he naturally went after the only other upper fish. I have never heard of this reversing, so the answer is not acquiring more Columbians, that cold be even worse. This species, which is Hyphessobrycon columbianus, has a reputation as something of a fin nipper at the best of times, and it is boisterous and active. Which is not so bad in a group of 10+.
Guppies might not be advisable with either tetra, given the fin nipping.
The same would have occurred in time with the five Buenos Aires Tetra (Hyphessobrycon anisitsi), they too need a group around 10=plus. Either species would be OK in a 40g as it is 36 inches (or longer?) length. I personally would not keep them together, given their very similar behaviours.
