How long has the tank been set up for?
What other fish are in the tank?
How long have you had the fish for?
How long after you got the fish did the problem start?
Did you add anything to the tank in the 2 weeks before this started?
Can you tell us about the other fish that died recently?
What does the fish's poop look like, and the thing that was hanging from its butt?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
Do you have separate buckets that are used specifically for the fish tank?
What is the GH (general hardness) and KH (carbonate hardness) of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).