My fish doesn't look very happy at all.

[GallumDrawnAndQuartered]

only had it about 10 days.

it's tail looks tatty and it's sort of hovvering by twitching it's tail quickly.

the other 2 look fine.

any ideas?

and it looked fine this morning.

fin rot?

does that come over them quickly? could be. should i take it out of the tank? i can get some medicine in the morning. will that be too late?

Did you cycle the tank before you put fish in? Take a look at this- new tank syndrome. A fishless cycle takes around 6 weeks. The other 2 might look alright now, it's always the weakest fish that shows signs of illness first.

For now I would do a large-ish (about 50%) water change to be on the safe side. You'll need a water testing kit to check on the water quality- make sure you get a decent quality liquid kit, the dipstick ones are massively inaccurate.

Could be bullying have you been watching them, to make sure the other two haven't been harassing it and nibbling it's tail?.