This is interesting to me for several reasons.
A) I keep goldfish and would like to know.
B) I'm an EMT and get a call for an MI (heart attack) at least once a shift.
In essence, what a heart attack consists of is this: the heart's main arteries (coronary arteries) do not deliver oxygen to heart tissue, and parts of the heart die. This leads to cardiac arrest (heart stoppage), and death. This could be caused by several things, and high cholesterol and triglyceride levels are only one cause among thousands.
It's totally possible that a goldfish could have a heart attack. What sucks is this: If fish do feel pain, then they experience one of the most painful medical experiences known to man. They hurt. Like really bad.