Hello

I have a pond of about 18 X 12 feet with a max. depth of 3.5 feet. The water
is clear and there is lots of vegetation, mostly
water lilies, irises and oxygenating plants.
There are about 30-35 ghost koi, comets and shubunkins of varying sizes up
to around 12" plus a load of small fish of various types from fry size to
3-4 inches - I would guess at about 60-80.
I have adequate filtration which incorporates a fountain at one end and a
waterfall at the other.
What's concerning me is that most of the comets and shubunkins and one or
two of the koi seem to spend all their time on the surface under the
waterfall ( they ignore the fountain). Other than gasping at the surface
they appear to be OK, there is no other unusual behaviour and they feed as
usual.
The weather (I am in southern England) makes no difference to their
behaviour.
I have bought some air stones which I am running for 3-4 hours a day. The
fish move away from the waterfall whilst these are switched on but the next
morning they are back to gasping on the surface again.
I would like any advice on whether the pond is overcrowded or if the water
needs testing. If the latter what do I test for and how do I interpret the
results.

I have thought for some time I would like to remove a lot of the smaller
fish so any ideas as to the best method/least stressful to the fish
would be appreciated.

Thanks in advance.

Conrad