Guppy Tail Disintegrating

[90JCR]

I bought a new guppy for my tank Sunday, when I opened the bag I noticed it had a huge inflated abdomen and soon began trailing long white poo. I came home from work today and it was just sitting on the bottom if the tank and it's tail had disintegrated!

I have no idea what's happening but all my other fish seem fine. Can anyone shed some light on this?
I don't have any large fish in my tank - just 3 stebai corys and 6 cardinal tetra, as well as one other guppy
 
I've checked ammonia, nitrate and the levels are all at 0

 

[fluttermoth]

How long has your tank been running and what are you using to test the water?

Are those plastic plants or real ones?

Oh, and how big is the tank?

 

[90JCR]

It's 20 gallons - tank has been running for two months now. I'm using API Ammonia testing kit and Tetra test kit to check nitrate
And the plants are plastic

 

[fm1978]

What's the pH and temp?

Guppies like slightly hard, alkaline and warmish water.

 

[90JCR]

80 degrees, and PH looks closest to 7.5 as far as I can tell

 

[90JCR]

I really don't think this guy will make it through the night. He can barely swim anymore. Does anyone have any advice?
 
I can't believe the rate in which his tail is shedding - it was fine this morning

 

[fluttermoth]

It does sound like the fish was sick when you bought it

All you can do is make sure your water is clean; do a large water change before bed if you can, and make sure the fish isn't being picked on; the other guppy, or even the cardinals, might have a go at it.

I'm dubious about your zero nitrate reading, as cycled tanks, unless they are very lightly stocked and heavily planted, should show some level of nitrate.

 

[ncguppy830]

yea many pets stores have stock tanks that might be overstocked or carry disease from fish that are sick thats why its important to quaritine.

 

[techen]

Big rock on fish if you don't believe it will make it. Better than letting it die overnight and infecting your tank.