All fishes died in the tank.

Did your sister change ALL of the water in the tank when she cleaned it? It is advisable to only change about 30% of the water per clean as the water develops good bacteria that keep the fish healthy, so never remove more than 30% of the original.

I am sorry to hear about the fish. I always just bought fish that can be alive in fresh water like beta fish. Minimal requirement needed.. just change water with your sink water, add drops from the store and feed the fish. My beta fishes lasted years, like 5! The fish is inexpensive also, but you can't mix the fish or else they will fight.

The trick is to change 20% of the water every other day. Never try to change all the water. If possible, change 10% of the water every day. The fish will have time to get used to the new water environment. The chlorine in the water is very poisonous to the fish. It is best not to put water direct from tap to the tank. You can either keep the water overnight or add a bit of anti-chlorine into the new water.

That's so sad. Maybe it's a good idea to just start of with a low maintenance type.

Yes, what is going on here is called "New Tank Syndrome".... the water wasn't cycled first and you didn't have enough beneficial bacteria in the tank (which lives in the gravel and filter)...so then you have Ammonia which is converted to Nitrites and then into Nitrates. Nitrates are okay, it means the cycle has completed. But the Ammonia and Nitrites are what most likely caused the fish to die, especially because the fish did not appear ill. What you need to do is run the tank for a while and then keep adding fish food to it, even though there are no fish in the tank. This is called "fishless cycling" and it will take a few weeks. Buy the $5 test strips and check for Ammonia and Nitrites. If both finally register at "0", then the tank is ready. *OR* you can ask the pet shop for about 50% of their aquarium water to add to your own. That will jump-start the cycling process and you can add new fish a lot sooner. But it's the fish's wastes (Ammonia) that usually end up burning the fish and then they die, unless the tank is properly cycled first. When I got 2 new goldfish, I already asked the pet shop if I could take all of their water home to add to my new tank, and they let me. You need that "old" water with all the beneficial bacteria to eat all the nasties out of the water in order for your new fish to be healthy and stay alive. Hope this helps. I've raised fish for over 20 years now, so I know my stuff. Feel free to PM me if you have any further questions, this is my area of expertise. (Assuming you are dealing with freshwater fish.)

Is your tank too cold? Where I live we have to have a heater in the tank, also water out of our taps has chemicals in the water so we had to put drops in the water to remove the chemicals. We also had a bubbler going to add air to the tank.

I think some fish are not easy to handle. It is easy for them to die even if it seems you have done everything wrong. This is why I try to buy some fish which can be easier to raise, such as the gold fish, guppies and so on.

Sometimes, drop in the oxygen level of the water kills the fishes.