Goldfish - help needed please!

[Mortini]

I'm looking after a friend's goldfish; she's away longer than planned and the water is going very murky. I don't think the hot weather is helping much, and am terrified that Harold will be floating on the surface if I don't do something soon.

I need to know what I have to do to clean out the tank / change the water - any advice much appreciated.

How big is the tank and is the filter working?

urmm. no filter - is that a terrible thing? Tank is on the small side about 30 x 20cm.

It's pretty dire, yes -- but obviously not your fault. I'm surprised the fish is still swimming tbh!

As a minimum, you are going to need some dechlorinator (Tesco and Sainsbury's do it, or maybe your friend left you some (hopes...)).
When you've got some then scoop out maybe 25% of the water; I'd give the tank walls a wipe down with plain kitchen roll at this point to get rid of some of the grot.
Don't use any detergent, soap, brillo pads or whatever.
Add correct amount of dechlorinator to fresh water in a (non-soapy) jug or bucket, check the temp seems similar to current tank water and slowly scoop/pour it in. You don't want to shock the poor fish too much by freezing it.
Then do it all again in a day or so -- and tell your friend to get a filter!

Thank you Lancelottie!
Off to aquarium shop this afternoon - hope my friend doesn't take offence if I buy her a filter as a "welcome home from holiday" present .

Poor thing looks a bit tragic all on his own.

Don't whatever you do buy it a friend to cheer it up! One goldfish alone is far too big for a tank that size (they're messy fish).

He'll have to make do with DS singing Twinkle Twinkle to him!