Fish have unusually low lifespans. Wit the exception of some such as koi.
Complete nonsense. Common fish like goldfish can live up to 10 years.
The fish probably died of:
- Poor quality water, you need to make sure there is good bacteria in the water which can take a week with treating it. Some places have chlorine in the water which kills it. 100% clean fresh water is bad.
- Shock; it should take 20 minutes to put the fish in the tank, by first letting the bags float in the aquarium, then by letting some of the aquarium water into the bags 10 minutes in. Fish can also die from a rough car trip-- good pet stores will put medicine into the bags to help calm them after purchase.
- Illness; the fish may have come from an infected tank, which may now have infected your tank. As a buyer its kind of your fault for not making sure.
- Temperature & water type; make sure the fish were fresh water. Some fish need salt water. Some die in warm water, some die in cold water. You can not mix and match.