He is known for songs of the genre Neues Geistliches Lied, particularly his setting of Dietrich Bonhoeffer's poem "Von guten Mächten".

Fietz wrote new songs in German for these occasions, inviting the audience to sing along.

[2][4] Fietz moderated a series for the broadcaster Hessischer Rundfunk, Lieder zwischen Himmel und Erde (Songs between Heaven and Earth), with up to 150,000 listeners.

[2][3] Fietz founded a music edition, the Abakus Musik Verlag [de], together with his wife Barbara.

He has collaborated with Ivan Rebroff, Clemens Bittlinger, Dieter Falk,[6] Eberhard Weber, Petula Clark, Uwe Kröger, Eva Lind, Bill Ramsey, Edwin Hawkins, Olivia Molina and Coretta Scott King.