HELP! WE have an established pond with 6 koi ( 6-10 inches) and a few comets. All have been very healthy until..... 1) my hubby and grandchildren introduced a dozen small gold fish a week ago (I know, I know, but the deed is done), 2) we have had a record setting heat wave and a tree that used to provide has been removed. The last four days, the koi have been hiding in a huddle by the waterfall or sitting on the bottom. They refuse to eat. One of them occasionally is listing. This morning 4 comets are dead and the koi are still not moving from the waterfall.

They don't appear to be gasping for breath. We have added extra aeration. No water change attempted yet. I have added several screens for total shade.

Pond size 600 gallons

Water temp is 74F.

pH 7

Ammonia 0

Phosphate .5 ppm

Nitrate 0 ppm

I can't see anything on these dead goldfish (aprox 4yr old fish here). What should i do to try and save the rest??? Could it bee from the heat?