Wrote an article on how important Hardness of your water is when keeping any commercially raised livebearer. If you're having problems with your livebearers getting stressed out and sick no matter what you do. This is a must read article.
http://www.tankgeek.com/2011/02/11/livebearers-weaker
http://www.tankgeek.com/2011/02/11/livebearers-weaker