Why do Roman Catholics consider eating fish on Friday a good work? Please forgive me if there are caveats or circumstances to this idea that I am not aware of.
The "good work" that one can do by eating fish on Fridays is penance. Perhaps it's hard to understand if you love fish and abhor meat, but that's not the case for most people. The following lines contain an excerpt of Keeping Friday, from the Canadian Conference of Catholic Bishops:

Jesus invited to carry the Cross and do penance for the good of the Church and the world. He died on Good Friday and, from that moment on, we share in the Lord’s Cross each Friday, learning to obey with Him, joining in His death to sin and evil. Every Friday is a promise from God that we who are baptized will rise again with Christ. We suffer and carry our crosses now, but we will share in Jesus’ glory if we are faithful by his grace.

How can we do this penance on Fridays? We may do it through these actions:


Abstinence from meat or some other form of food, drink or entertainment [Note 1: this is where the "eating fish" custom comes from; you exchange one for the other].
Prayer: take part in a service of worship with others, or pray with our family, or spend some extra time in personal prayer.
Good works [Note 2: meaning concrete exercises of charity, because abstinence and prayer are also "good" works]: do good to others by visiting the sick or aged, helping those in any need, or by contributing time or money to a work of charity.