any suggestions

Hi all.

My girlfriend has had two goldfish ( a comet and a Shubunkin) in a small (5
litre) bowl for 18 Months. Three weeks ago I bought her a larger (25 liter)
aquarium for christmas which the fish were placed into. They seemed happy a
lot healthier in the new tank. One week ago four more fish (a shubunkin, a
fantail and two smaller fast swimming ones that I cant remember the name of)
were introduced into the tank. The local pet store informed us that there
would be no problem doing this. All six were swimming around quite happily
for the week. However yesterday we found that both the original fish had
died, side by side. Neither had shown any sign of disease prior to this. Is
this sort of thing common?

Thanks,