HELP! WE have an established pond with 5 koi ( 6-10 inches) and a few comets. All have been very healthy been very healthy until..... my hubby and grandchildren introduced a dozen small gold fish a week ago. ( I know, I know, but the deed is done). for four days, the koi have been hiding in a huddle by the waterfall or sitting on the bottom. They refuse to eat. This morning 4 comets are dead and the koi are still not moving ar

ound. They don't appear to be gasping for breath. We have the waterfall and extra aeration going. No water change attempted yet.

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???