This was one of just a couple sick fish you had. Sometimes fairly minor things will set off older ones. You mentioned red in the fins, often a bacterial thing. It is soooo easy to let routine maintanence slide a little during the furiously busy holiday season. (Sometimes the holiday season goes 365 days/year here.)

The youngsters should be ok. You may want to treat with a "proprietary anti-protozoan remedy" with them. I would at least try to be more militant with water changes. (Easier said than done for me too.)

You mentioned a generic guppy disease in an earlier post. Over the weekend I ran across the mention of a "guppy disease" caused by a ciliated parasite called Tetrahymena corlissi. Evidentally protozoa of that genus are common in a tank. The one species can be a problem in ripe water and serious cases are usually too far gone (internal damage) to cure. Signs of it, according to B.B. & E, are small white patches on the skin or around eyes of livebearers.