Pardon me if I overlooked anything here:
I would say that fired clay would be fine as long as it wasn't glazed. The fired clay, as others have stated, should act like other ceramic media that you can store buy.
Respectfully, FireMedic
P.S........if you are using small pieces in your filters, make sure they don't collide to much/at all. The constant collisions/cavitations could result in ceramic dust entering your impeller sumps and wreaking havoc on the magnets,bearing etc.