Fish don't become fat.. their stomachs expand (and should only to a point) right after eating.. but a constant overly large roundness means there is health issues.
I tend to fast differently when I decide to fast my fish.. I will feed in the morning, then skip dinner that night and skip breakfast the next day.. then start feeding once more that evening. So they have time to fast.. but are still technically fed daily.
But that's just me.. you don't really need to fast them, but I do occasionally because they often do in the wild themselves. The higher quality food you feed, the easier it is to digest/process and less of a chance of bloating and constipation.
Live foods, high quality pellets (I prefer Omega One, then New Life), and home made foods are the top of my list for feeding.