Female Rainbow Platy

Hi,
I have a Female Rainbow Platy that I got 6 months ago she is about 8 months old right now and she is losing wieght. She still has all her color and all her scales, but she is getting much, much smaller and I'm worried about her I've tried stress reliver and ick clear tablets and I just put her in isolation to see if she'll get better but I wanted to get a profesional opinion. I have two Red Wag Platys, 2 Albino Corys and 4 Longfin Rosy Barbs and they are all growing and healthy as can be I have two other rainbow platys that aren't growing very fast but they are still healthy. These are my first fish and I'm kinda worried I'm doing somthing wrong or somthing.

[seattlemom2plus]

How are your fish now have you maintained your tank?