Yup. They are by no means easy to breed, so don't expect to have fry unless your tank is in pristine condition or you're very lucky. Usually fat females signify the arrival of the spawning season, but I find it weird that out of 8 cardinals you would only have 1 female. Maybe it's just a little pig and eats a lot more than the others. Watch them at feeding time and you may find out.
