poorly guppy

[fisharefriendsnotfood32]

Have started a new thread hoping to get some advice.

Had 4 guppies for a week. 30L tank have done a 1/3 water change this week not with syphon just taken out with a jug and added water conditioner. Will do a syphon clean tomorrow. Tank cycled.

After 2 days of having them I noticed one was on the bottom of the tank quite a bit. The others are always in a group and are happily swimming about.
No signs of bullying. The guppy does seem a bit on the skinnier side and always has white poo hanging from it. It looks like cotton.
I've not seen it eat for 2 days.
Today it's just hanging around by the filter suction cups and I've noticed the back end is arched downwards slightly and it's tail is hanging down a bit. It's hard to explain but it looks different to the others.

Is there anything I can do for it? Does it sound like its on its way out?

Totally gutted as its only been a week. I don't think it's anything I've done or tank condition as water test is spot on and the others are very active fish.

I've attached a picture as it easier than describing it. The poor fish seems to have lost its colour too and now looks pretty bloated at the front.

The picture by the black is the poorly one and I've attached a picture of one of the others for comparison. These two fish were different colours to begin with so don't worry about that difference.

Pics

Just updating this incase tree sparrow spots this and can tell me if I have done the correct thing.

A hour ago I noticed red flashes on the guppys tail and the back end of the tail looked a bit jagged.

I tested the water this morning and had readings of
Ammonia 0ppm
Nitrite 0ppm
Nitrate 10ppm

After googling I came to the conclusion it was a bacterial condition . My daughter said the thread poop has been hanging from that particular fish since the day we got it all the other fish have more solid poops.

I have done a a 3/4 water change and have set up a hospital tank (20L bucket) and put poorly fish in as I can't tell from google if the other fish may get it.

If you see this tree sorrow have I done the correct thing? Is there anything I can do to make sure the other fish don't get it?

Just to update incase anyone finds this thread with the same issues.

The poor little fella died overnight.

My daughter reminded me that it won't last long without a heater, so every couple of hours I was taking out water and topping it up with hot to keep the temp to 25oc. So I was setting my alarm to keep topping it up but it died in the early hours.

I am going to get one of those plastic tanks with a lid next time I'm in pets at home and a spare heater incase this ever happens again.