Fish will be spawning. Look for fish that have managed to jump out of the pond and watch fish closely for injuries sustained during the spawn.
Monitor water temperature, both air and water, in the morning. Once the water temperature stays above 45° F you can add the floating plants to your pond (like hyacinth and water lettuce). Remember it is the early morning that has the coldest water temperature and it is that temperature that will kill your plants.
You will find the largest selection of new pond fish from mid month through June to add to your pond.
This is a good time to make modifications to your pond and general structural upkeep.
Watch for frylings (baby fish) and give them a protected area to thrive in if you want to keep them as the adult fish will eat them.
Do not spray pesticides on or near the pond. These are deadly to fish.
Spring is the hardest time of the year for your fish. They are coming out of their hibernative state and their immune system is not up and running yet (not until the water temperature is constantly above 62°F), but the parasites and other diseases are. Watch your fishes activity levels and look closely for signs of parasites and diseases. Remove sick fish from the pond and treat it for it's ailment. Do not use the pond water as it contains the very organism that made the fish sick to begin with. Don't forget to treat the hospital tank water for chlorine and try to get the water temperature as close to your pond water as possible to avoid shock to the fish. It is always a good idea to keep more than one fish in your hospital tank as most fish are schoolers and to have them alone adds stress to an already weakened fish.
Things to do:
The first part of April sees big temperature swings. Do not feed your fish just because the temperature was 65° F for a day or two. They are still fine without it. Wait until temperatures stabilize later in the month.
It is a good time to remove all the dead leaves and organic matter. This prevents the wrong organisms from getting a food supply and proliferating and harming your fish.
It is a good time to repot your plants (except spring bloomer like water iris).
Start your filtration system once the water temperature is maintained above 40° F.
You should replace 40% of the water in your pond to help cut down on organic matter that will feed your algae boom which usually starts mid month.
You will also see an ammonia spike followed by a nitrite spike as the bacteria come to life. Monitor the water quality weekly and change water out as needed