No offense there. You haven't kept any fish alive for longer than three days. Obviously you were doing it wrong and what you know is wrong.
Yes, some fish are easier to keep than others, but not by much. If there is ammonia in the water, all fish will eventually die. So called "hardy" is just the matter of which one last the longest in a (unsuitable) environment. Without a filter, the ammonia will build up. There is no question about it. If you did not cycle the filter before adding the fish, ammonia will also build up... always... 100%. All "hardy" fish will die sooner or later to the toxic. If you do things right, it does not matter the fish is hardy or not since they will all live.
I very much doubt your cousin had kept the same guppies for years. She probably replaced them many times since guppies are cheap. Fish bowl with no filter nor heater have no chance to keep 8 guppies for years. Guppies are known for breeding fast too. If she indeed kept 8 of them alive in the same bowl for years, there should be hundreds if not thousands by now.
Quote: I have a big tank with clean water, the correct ph, filters, and the whole bit and the fish dies in a couple days or weeks.
"Big tank"? How big? In term of gallon or liter? The term "big" does not tell us anything at all. Someone might think a 5-gallon is big. While others will think a 180-gallon is big.
"Correct PH"? What exactly is correct PH? Different fish species live in different environment with different PH. What exactly PH did your water have?
Filters? Which exactly filter did you use?
From the way you have been telling us, you are a novice fish keeper who hasn't really started to learn the basics yet.
If you would like to keep fish alive and healthy for years, you need to listen to those who have been doing just that.
