I managed to poke myself in the eye with the handle of a sack barrow last Thursday and, apart from the pain, I had occasional flashing lights and wavy vision so I've had to spend the past few days seeing three different eye specialists to make sure that I hadn't damaged the retina. It's all ok apart from the embarrassment of a black eye but maybe sack barrows should have a warning label attached saying that they shouldn't be used without eye protection!!!
The way I would rationalise the situation of your koi being skittish when you turn on the air to the aerated drains is to think about what the fish see. Bearing in mind that they have no concept of what aeration is, if you don't have any other aeration in the pond, when you turn it on, suddenly there will be a big bubbly swirly thing that they've never seen before and they naturally will be nervous in case it eats them. They may become used to it over time but I don’t like causing koi unnecessary stress so I would suggest having it on but initially with the air turned right down to literally just a few bubbles. They should soon come to ignore it and you could try turning up the air by very small amounts every few days until it’s fully on.
Personally, I don't like aerated drains because the surface bubbles in the middle of the pond make it difficult to see the fish. I wouldn't try to persuade anyone else not to have aerated drains if they don't mind the surface disturbance because they do make bottom drains more efficient. Also you, and everyone else who wants to see the fish, can always turn off the air (and remember to turn it back on again) every time you go to look at them - it's just personal preferences.
I'm not in favour of having no aeration at all in the pond with just plenty of air in the filters so I prefer a large ceramic air disc hanging vertically in a corner a few inches from the bottom. That way, there is nowhere for debris to build up on the floor underneath the disc and there is massive aeration but only a quarter of the pond is too opaque to see through. I can live with that and, in a pond with good circulation, the highly aerated water in the corner spreads quickly and evenly throughout the entire volume. Large ponds can have discs in two or more corners, suitably positioned so as to only cause acceptable surface bubbles.
Maybe you could have both; air discs and aerated drains. That would give the pond good aeration all the time and the koi would become used to the bubbles and be less concerned if the air to the drains is turned on and off.
