I've had my cardinals about 1.5 years now. The first year they bred like crazy.... then all the sudden they stopped. I've had no babies in about 6 months now. And just like you guys, I never would see molting at all.
I don't mean to hi jack your thread, but any ideas as to why the would suddenly stop breeding?
I just checked the water parameters a while ago.
0, 0, 0 amon, trate and trite.
Mine also rarely eat man made foods. Just algae from the rocks.
I'd love to know what happened?