Spring (when it follows a real winter) is the absolutely worst time to do
pond care. The fish have not been fed, their immune system is down and the
"cooties" wake up before the fish's immune system does. The stress of them
being moved out of the pond, or even just "chased" in the pond during this
time will lay them open to disease. Here in zone 5 spring comes on March
21, but a typical unheated pond wont be over 55o until late April or early
May. And by that time the fish will be spawning and stressed and eggs are
very susceptible to bacterial infection while in the female. So it is best
to wait until spawning season is over.

It is not a good idea to buy fish in spring from areas where spring follows
a real winter either. "summer fish" already have a good immune system
online, spawning is over and they have recovered.

Fall is the best time to do pond care that involves more than lifting
plants in and out, or cleaning external filters etc.

INgrid