fish dying

thought i would draw on the collective wisdom of this group.

i have a 300g inground pond with a 45 mil liner that has been running
since april of this year. it has a 950g/hr mag pump with the discharge
split into a venturi and a bioforce 1000 bio-mechanical filter. it has
been home to 6 - 5 inch comets during this time with no probs. been
doing 10% wcs monthly and did a 20% wc 3 weeks ago when i cleaned the
filter pads (rinsing the pads in water taken from pond for the wc). i
use amquel+ for water top offs and wcs (treating the water taken up in
30g plastic containers and aerating before adding to the pond). the
pond sits under a patio cover in the back yard and is shaded
throughout the day.

the prob:

i lost 4 fish in the last 12 hrs. with no signs of flashing or
abnormal behavior. the four started going belly up within hours of one
another. as i saw signs of them struggling to hang on (swimming
erratically, etc, as dying fish do) i placed them in a 30g container
with freshly conditioned water and a powerhead for circulation. they
died hours apart. ran tests at first sign of distress:

nh3- 0
no2 - 0
no3 - 10
pH - 7.8

i keep fresh and salt tanks and know losing fish sometimes happens.
but i only lose 1 every so often - never a fish kill like this. i
think i've done everything right (including adding a large bag of well
rinsed activated carbon to the return of the pond last night to
remove possible pollutants). i just looked in on the remaining two and
they're swimming around fine - no signs of distress.

i know this is long but am hoping to get some possiblities of what
happened.