Q. 1. Is it true that Catholics must eat fish on Fridays?
A. 1. The Catholic Church does not teach that Catholics must eat fish on Fridays. Canon Law states: "Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday." [Canon Law # 1251]
For centuries, prior to Vatican II, because Catholics could not eat meat on Fridays, they substituted meat for fish in their meals. It was not necessary to eat fish; Catholics could have denied themselves both, meat and fish if they had chosen not to eat fish.
It should be noted that, many of the Catholic practices that find their origin in their early days of the Church, have been proven in the last few decades to be of benefit to a person's health. For example, fasting one day a week is beneficial in controlling one's weight. Eating fish helps to lower cholesterol and prevents heart attacks from excessive eating of meat.