There is just so much water in the pond ... so much energy from the sun hitting the pond ... so much plant and bacterial respiration at night in a fed (eutrophic) pond that this is a very hard row to hoe. Rather than trying to keep them alive in the pond ... may you could make a smaller tank ... say like 10x5x3 ... where you could keep the temp low and the oxygen up ... using just enough water exchange to keep ammonia and other toxics at acceptable levels. Maybe you could try this year to head start some stocker brook trout through the summer so they have larger weights in the fall. Next year you could try to partitioning so that a few overwintered fish and next falls crop can be grown simultaneously?

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers