What is the history behind eating fish on Friday?

I had a former Catholic tell me he was born into a Catholic family but when they started changing things like the fish on Friday thing and making annulments as easy to get as a divorce, he left.
Did the Catholic church ever tell Catholics they had to eat fish on Fridays? I thought it was more a matter of giving up meat rather than having to choose fish.
Chukm8

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Where can I find a history of Friday abstinence from meat?**
*]Why is Friday for not eating meat and not another day of the week?
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