I can tell you where the tradition of fish came from. The Church at one point stated that because Jesus died on a Friday, we should do penance every Friday. They went a step further and dictated what the penance should be - do not eat meat on Friday. Originally, "meat" included "fish".

Then the fishermen along the Mediterranean Sea (and there were plenty in Italy) complained that it was hurting their business to much, and they couldn't make a livelihood fishing because of that rule. So the Church re-defined "meat" to exclude "fish" - and fish were allowed!

We're not ordered to eat fish on Friday. We used to be prohibited from eating meat on Friday - and still are during the Friday's of Lent. But we can eat any non-meat food we want. There's no requirement that we eat fish.

Radar