Sick Guppy

[havoc_crazy]

My newly established 10 gallon tank (two weeks old, was cycled 4 weeks before adding fish) has 5 guppies and 3 corydoras. One of my new guppies from Petco doesn't eat and hangs around the bottom of the tank alot. It also has a long clear tube (a few inches long) coming out of its rear end. The rest of my guppies seem to be doing the same. Can someone help me find out what's wrong?

pH: 7.6

tank temp: 74 degrees

Volume and Frequency of water changes:15% every week

Tank inhabitants:guppies and corydoras

Recent additions to your tank (living or decoration):2 guppies and 1 albino cory

Exposure to chemicals:Amquel and Stress-Zyme

(Edit: Forgot to note that my friend posted this, so I can't access the tank to see. Ima ask him to reply to your questions)

 

[WILDER]

The tube coming out dosn't sound right, can you see any prutruding worms from and anus of the fish, just the one fish with the tube hanging out, heard of it before and the fish don't usually make it, sorry.
As for the others are they laying on the gravel to one side or resting on things, what does it look like when the fish go to the toilet.