Sick goldfish?

[MonoJoker]

Hi,

I recently got given a tank setup with a pair of approx 8" long goldfish:

http://imagestore.ugbox.net/image/620be724540113b496bf91f701bbee6d.jpg

Recently the large orange one of the two has started swimming on some weird
angles. Also it appears quite lethargic at times simply floating amongst
the weed at the top of the tank (rightside up I should add). At feed time
it appears fine and swims around happily (sometimes sideways) gobbling up
the food. I change the water every week, I have recently changed the filter
and added new carbon to try and keep things healthy for them. I stopped
feeding them flakes and tried sinking pellets but doesn't seem to make any
difference. The other white and red fish is fine and swims "straight".

Is this a cause for concern or do I just have an eccentric fish?

Any suggestions would be appreciated as they're great looking fish and I
really want to keep them healthy.

Thanks
Liam.