What is a memory span of goldfish?

Goldfish have a memory span of about three seconds. This means that they can remember something for about three seconds before they forget it. This is a very short memory span, compared to humans who can remember things for much longer periods of time.

My fantail goldfish has lost its scales and stomach gone very big moving wierdly staying in dark areas which are covered by plants .whats wrong with it.HELP ASAP?
Where do you buy goldfish?