Moor floating on side

Hi all, my moor appears to be floating on the top on it's side most of the
time. Occasionally it has a swim about (lethargically) but seems to be
demonstrating the signs described he

http://experts.about.com/q/1472/3627718.htm

The advice is to feed it peas, but since it's not eating, I can't very well
force the pea down it's neck! When I'm feeding the others, they are all
fine and demonstrating no signs of illness. The moor knows I am feeding
them and lazily sucks the surface but makes no real attempt to hastily get
the food like it was last week and ages before.

I did a water change over the weekend and it has been like it since then.

Any other advice appreciated.

Thanks.