So, my sister was given a betta fish last week. She really didn't have any idea how old he was or anything, but he seemed fine. She kept him in a one gallon bowl for a few days and he made a big bubble nest and ate well. She then moved him to her 10 gallon, cycled, filtered tank. It wasn't heated, but the water was not getting too cold. He was only in there a couple of days. She said last night he did this cough-like thing and looked like his color had faded a little, and then this morning he was dead.
Is there any way to tell what could have happened?