fish problem

Question
hi my name is jeeva, i want to ask you question. I have a 25 gallon fish tank with small tetras and dwarf gourmi which i bought about one month ago. He was doing fine, eating well and active and couple of days ago couple of days i notice he was at top gasping for air and he was very active and also saw a white string on poop was hanging. he eat a little. last night he was almost passing out, and today morning he died. I don't know the cause this. I tested my water level, it's fine. And all other fish are fine, doing well. I been feeding flakes and once a week blood worm. I don't what happen.

Thank you

Answer
Couple things I would need to know in order to really give you an accurate idea of what may have happened, what was the temperature in the tank at the time you had these issues, what if any kind of an air stone or other source did you have, was there anything abnormal about the fishes appearance aside from the fecal observation? Once I get this information I will be better able to give you an accurate answer of what the issue may have been.