In article , Magwitch writes

Snipped

My parents have 15 koi in what is little better than an old farm pond ‹ they
all seem fine, some about 18* long, breed and they've been there for about
20 years. So long as there's enough vegetation to hide from the herons tehy
shift for themselves. Are you sure all this equipment is really necessary?

Depends on the size of the pond, plants and number of fish, ponds can
become self sustaining if the conditions are right, I have gone the
equipment route but my neighbour just leaves his natural and the Koi
seem fine, my brother in law has gone to the extremes with equipment and
treatment and lost a lot of (expensive!) fish
--
David