I have to tell you, we have only goldfish and no desire to have Koi. We started in 1999 with 12 feeders, we have 1 remaining and it is now 7" long. We added 2 gold fancy fins, 2 shubunkins and 4 comets later in 99. Everybody made it thru the winter of 99-00 which made us very happy.
In the spring of 2000, the pond had a population explosion, now we have 60 or so goldfish of all
blends of colors. We figured that nature and winter would thin out our herd. Well everybody made it thru the winter of 2000-2001.
Now we have a problem, how am I going to thin out the school. First attempt was with a 10" net, do you know how fast those 2" fish are, no catch. Then I got a minnow trap, got 6 on the first day. They got smart and I didn't get another one. I got a salmon net, it was the biggest I could
get, replaced the net with a fine mesh. This did the trick, got 15 in the first scoop, then they were gone, not a fish in sight. The next day I got 12 more, so now 31 of our school are 3 doors away in a new 2000 gallons pond
The pond freezes over in the winter and I use a cattle trough heater to melt a hole to vent gases. I run the heater about once a week, just to open the hole.