I never found mine to be piscivores, although I never kept them with anything as small as cardinals.
I was warned before I got mine, that as adults they would need over 200 gallons, and if not given that kind of space they get rather nasty, with each other, and other Geophagines.
I poo-pood this thinking my 6 ft 150 gal would be sufficient.
As it turned out at about 7", i was wrong and those who warned me my tank was too small, were right..
At maturity, the largest of about half dozen, subtly stressed the other more subordinate Acarichthys to death.