Having fitted my air pump in the house boiler room, I was very happy with
the
performance and so were the fishies. However a couple of weeks ago,
the boiler had a bit of sideways moment and pumped out a lot of CO
which the air pump duly pumped into the pond. Sudden fish death
swiftly followed with the symptoms of loss of colour and reddening
of the gills. Luckily for the human occupants as well as the rest of the
fishies, the accompanying smell that the boiler emitted meant that it
could be fixed rapidly and everything is now back to normal.

So just another fishy tale and something else to worry about when deciding
where to mount the air pump.