Beacause of the time of year (and asmume you do not heat your pond), the 'bister' may be carp pox.
Carp pox is generally a harmless virural disease that will most likely disappear when the water increases in temperature.
Important that you get a proper diagnosis, in the meanwhile keep a very close eye on the fish.
Posted on Mar 24, 2006, 3:31 PM from IP address 213.166.17.22