I have cardinals too, and I feed them usually once a day. I alternate between high quality flake food and store-bought frozen foods, which are usually brine shrimp, blood worms, mysis shrimp, or daphnia. Two or three times a month they might get live brine. This goes for all of my fish.
I haven't found much of a reason to deviate from this, except in the case of rearing fry. I might feed fry two or three times a day with frozen baby brine/daphnia or finely crushed flake.
I feed a variety of foods: pellets, flake, frozen, dried. 4 times a week I feed once a day & 3 times a week I feed twice a day. On occasions I feed live brine/blackworms. My bottom feeders/algae eaters get feed once to twice a week.