Help - My fish won't feed

If anyone can help me with this I'd be most grateful. I have a 3,000 gallon
pond with 7 Koi about 20-25ins each, normal bio filter with sediment chamber
set up. Due to the recent hot weather (water temp 23c here!) I decided to
treat the fish to some extra oxygen so I bought an air pump and two
airstones, installed it all and switched it on expecting the fish to enjoy
this extra boost. What did they do? Swam to the bottom of the pond and hid
under the venturi to get as far from the airstones as possible. A bit of
moving and tweaking and they are now swimming around normally. Except they
have stopped feeding - It's been nearly a week now of the hottest weather
and I've had to fish out every pellet that I've put in - They don't even
rise when the food goes in. I tried switching the air pump off for a day but
that didn't make any difference to them either. There is nothing else edible
in the pond except a very small amount of residual blanket weed from a pond
balance treatment two months ago which they nibble at occasionally as they
always have done. All other factors are the same: Perfect Ammonia, Nitrate
and Nitrite levels. PH around 7.5 - 8.0 (where it normally is) same food fed
in the same place at the same time as normal. I understand that Koi are
sensitive, but surely after a week they would get used to the airstones
(they're turned down low so they don't create a massive current). Any
suggestions?