Fish Stopped Eating
 

Fish had been doing fine (2 sarasa comets, 2 shubunkin, 3 koi), all
about 4 inches and healthy looking (at least to my eyes). They were
eating like little piggies, practically out of my hand,until about three
days ago. Now they seem very shy, swimming very quickly to hide under
the plants from one side of the pond to the other. Water quality:
ammonia=0, nitrites=0, nitrates=0, pH=8.5 and pretty stable, GH =5, KH =
5. Water is clear although somewhat tinted brown. Water temp = 70.
I've been doing about 10% water changes once a week. I know the pH is
on the high side but my tap water is pH=8.4 and the fish have been at
this pH for about 6-7 weeks now and had been eating fine.
Pond is 500 gallons, with waterfall, trickle biofilter with UV and
biofilter media in waterfall.
Any ideas on why my fish have stopped eating? I haven't changed
anything in the pond.