Okay, lets look at your claim that Rummy-nose has reached its life-span at 2 years in aquarium:
"Lifespan for the rummy-nose tetra in the aquarium is usually 5 to 6 years with careful maintenance. Exceptional specimens can live for more than 8 years."
"Approximately 5-6 years."
"The average Rummy Nose Tetra lifespan is 5 to 6 years in captivity.
However, some owners have managed to help these fish live up to 8 years (great genetics comes into play when exceeding 6 years)."
The lifespan of Rummynose in wild:
"In the wild, Rummy Nose Tetra lives typically for around eight years. However, in captivity, they generally have a lifespan of about 5 to 6 years."
So, no, the fish did not die of old-age.
Also, lets assume this species reached its maximum age at 2 years ( it doesn't, but lets just use it as a hypothetical) the likelihood that all would die of old age in the same week is highly improbable. Some would be stronger than others and would be susceptible to disease over a period of months, not days.
It is probable, however, that their immune systems are depleted from long-term exposure to improper water parameters and all were susceptible to whatever pathogen has killed them. . Long-term stress is the leading cause of disease in fish.