"Very"? No way. They're not as delicate as most people assume, but I'd reserve the term "very adaptable" for such species as Paradise fish and danios. I hate discussing "tolerable" ranges, since that involves a spectrum of arbitrary lifespans set by the experimenter and requires lethal outcomes to find those parameters. However, wild cardinals probably would tolerate pH in the 3.5 - 7.5 range, hardness to about 12 or 14 dGH, temperature from mid-70s to mid-80s. The less extreme (except for softness), the better.