Question
I have a pond that is basically in the shape of a triangle.  The center is approximately 3' deep and shallows out  to 1' towards the ends.  I had been successful in keeping goldfish for 2.5 years.  The first group of fish became sick and slowly all died off.  I checked and corrected problems with the water quality.  I replaced the fish and they seemed to be doing well.  This year, I started out with approximately 12 fish, during the summer they mated and some of the fingerlings actually made it.  

 That summer, my husband planted an ornamental pear tree next to the pond to give it some shade because I was having difficulty controlling the algae.  It previously had direct sunlight for most of the day because it is on the west side of my house. I should also mention that I live in Illinois so we have some pretty harsh winters.  

 I place a heater in the pond after we close it for the winter every year.  For the last two years, each winter the some time in February, the weather warms enough that the pond thaws to the point that the heater is not needed. During this time, the goldfish will come to the top and become active again.  Then it becomes cold again and the pond refreezes.  I do leave the heater in.  When the pond thaws permanently in March, I find most or all of the fish have died.  This winter, a few of the fish were actually frozen in the ice. (never happened before).  I just started the pond back up and I observed that I do have one lone survivor.

 My water quality is typically within normal ranges.  The one downfall is that I always seem to have a above normal pH.  I have tried using the stuff they sell in the local store but I have a fear of making it worse.  

 Why are my fish dying during the mid winter thaw?  Is there not enough oxygen, the nitrates to high? Any suggestions would be helpful.  Thank you for your time and patience.

Answer
Dear Jill,

Thank you for your question and sorry to hear the fish keep dying.
I can give some things that could be wrong or give problems.
First, as I think you have in under control: the waterquality especial PH, ammonia, nitrate...
Also good health of the fish is important with good feeding in de autumn.
Then the depth of the pond is advisable to have approx 1 meter deep for domestic fish and up to 2 meters for koi.
Other thing is to try to keep the temperature of the water smooth.
I mean that you need to try to have A stable temperature and let is smoothly get lower to the winter and the same for spring. The best way is to cover the pond and keep it low heated with A regulator so the temperature of the water is approx 9癈 during the entire winter.
When A pond completely freeze it is possible that the water was to cold or, when there is people running around the pond they tend to come up and get stressed and freeze to the ice!
If the pond is frozen you better make A hole in the ice (placing A warm bottle on the ice) and lower the water with approximate 1cm so there becomes A airchamber between the ice and water.

Also best way to get rid of the algae is waterchanges, good filtering and UV.

Hope it helps?!
Kind regards and good luck,
Raf