No matter what kind or brand of fish tank you have, there are a variety of reasons why your fish might die. There are a number of reasons your fish could be dying. Could be due to a lack of food, not having the right food, and not having the right water temperature. Other issues could be having dirty water. If the water doesn't have the right acidity level, and so on and so forth. There are literally dozens of reasons why your fish might be dying.