upside down swimming

[Loshad]

one of DS3's fish - coldwater goldfish type keep swimming upside dwon, behind the pump/filter, he alomost looks as if he is stuck, but as soon as i lift the lid he swims off, but still oddly - more vertically/upside down than normal.
Can i do anything to help him?

What sort of fish is it? Lots of different things can affect their swim bladder (the organ that helps them balance)- some can be fixed easily, some not so easily.

Can you tell us a bit more about the fish and tank set up? How big is it, how many fish etc. and water test results if you have a testing kit.

tank is huge, water is about 20 cm deep, and then it's 60 cm long and 30 cm wide so lots of water, only 2 fish in there, don't have any water testing results sorry. Having googled on net i have : replaced 25% of the tank water (with treated water), cleaned the filter, and fed them some shelled peas.
I'm afraid i don't know what sort of fish he/she is ; looks a bit like a comet but that is just me comapring pictures on the net.
thanks

Hello.

It's hard to say without water test results but a bit more info might help to figure out what the water quality is like.

How old are the fish, any ideas? If you bought them from a pet shop then how long ago was it? How big are the fish now?

If they are goldfish then the tank (brace yourself) is too small. Goldfish get very very very big and if the tank is too small then it can cause all kinds of health problems. This is about tank sizes and life expectancy.

How did you clean the filter?

Eau, thank you, s/he is much better and swimming normally.
The fish were from a pet shop about a year/18 months ago, they are still tiny, do they need a pond then? (aargh cats might eat them )
the little one - the one who has been poorly is about 5 cam from mouth to end of fin, the bigger one is probably about 8 cm in length - he has a big belly though as well.
I washed the filter with some of the treated water.
Gosh this is much more complicated than you might think isn't it - i have 2 goldfish at school as well, also in a similar sized tank.
If i was to get a water testing kit what is a good one to buy please?

If you get a liquid kit rather than a dipstick one then they are much cheaper in the long run and I find them much easier to use too. The API mini master kit is a popular one, they go for around £20 on ebay. You need to be able to test for ammonia, nitrIte, nitrAte and pH at the very least.

Common type goldfish, comets and shubunkins do better in a pond although they are fine in a tank if you get one big enough. Fancy goldfish (the fat, round ones with long flowing fins) need to be kept in a tank because they are too delicate for the great outdoors.