I always see adult males always keep their distance from one another. You got lucky to get a pair right away. I have had to purchase up to 6in a group to let them sort it out . Once a pair forms it’s best to remove all others . I then wait to see if another pair forms once they have been separated from the original pairing . . It is a good idea to remove the male after he has released the free swimming fry to give him a chance to recover from not eating . The female will be ready to spawn as soon as he releases and then he will not had some time to eat and recuperate .
I have had difficulty in getting some cardinals to eat whether they were wildcaught 20years ago or recently aqua-cultured . Often they don’t consider food falling from the surface as being prey . They will watch it fall or they will move toward it for inspection and turn away . They could literally starve with me dropping frozen food all day long just because it wasn’t live . The fix I did was to buy feeder guppies and drop them in . But warning, guppies need to be transitioned otherwise they drop in and quickly do a death spiral . I was lucky to have gotten florida guppies which I could drop right in without transition
