I wonder if it would be a good idea to treat all of the girls. Both internal parasites and bacterial infections are contagious. What do you think, Callistra?
If you have live plants or other inhabitants in the tank that are sensitive to those meds, both Kanaplex and metro can be mixed in a solution with water or garlic juice and you can soak the pellets in it (except for New life spectrum as they don't soak well). You could probably try feeding metro & Kanaplex mixed for two weeks and then metro for an additional week (Kanaplex can be treated for two weeks and internal parasites need treated for three). Kanaplex is a gentle med and metro I believe is not too harsh, either, since its pure. People don't usually recommend treatment if there are no signs of sickness, but I often see people recommend treating a whole tank if one fish is sick, and I think that would be a good precautionary measure, anyways.
You could also use IAL (if you don't already) to boost the remaining girls' immune systems or you can get VitaChem and use it along with tank water or garlic juice to soak the pellets, as well as adding it directly to the water, because it, too, boosts the immune system.
I really hope that now, you can prevent more girls from getting sick, especially since you have both metro and Kanaplex. If you decide to treat them all, hopefully no others will be lost. Of course, only time will tell.
I wish you and your girls the best of luck.