What do the test strips say? Accurate or not, they will give you a better idea...
The fish sound like they are dying quickly?! Is it just tatty and sore looking fins/tail or does it rapidly spread all the way up their body and half paralyse them before they die?
Like this?
http

/www.google.co.uk/imgres?q=guppy+finrot&hl=en&biw=941&bih=512&gbv=2&tbm=isch&tbnid=JVgFWKV3OBO8ZM:&imgrefurl=http

/www.wetwebmedia.com/fwsubwebindex/guppydisfaq2.htm&docid=zB5GlyB-aK3DWM&imgurl=http

/www.wetwebmedia.com/CyprinidontiformFishPIX/Livebearers/Poeciliids/CIMG5737.jpg&w=300&h=208&ei=WU-qTs3_IcWv8QOE7dmnCw&zoom=1&iact=hc&vpx=93&vpy=149&dur=1406&hovh=166&hovw=240&tx=135&ty=82&sig=110957000062565089217&page=1&tbnh=130&tbnw=150&start=0&ndsp=8&ved=1t:429,r:0,s:0
Or like this?
http

/www.flickr.com/photos/feliciamccaulley/3956863083/
(though not necessarily fluffy, thats a secondary problem)