dying guppies, help!

Okay, I posted a few threads back about weird algae growing in my tank and a
lot of my fish dying. Well, I have noticed that most of the ones that have
died have had something that seems to be protruding from their butts. I know
this is kind of gross but I couldn't find anything about a remedy for a
sympton like this on the internet. The only thing that I found was something
called Nematoda, which I think is a sort of parasite but I'm not sure if
this is a correct diagnosis or how to cure it if it is. Presently I have
only two or three fish (guppies) that are showing signs of this and glancing
off the gravel. Could anyone please help me with this? I am going to give
another dose of quick cure and change a couple gallons of water for now, it
is the only thing I can think to do. Any and all suggestions appreciated,
thanks.