Hi, I am the OP here, under a different name. I use google and another
newsreader and the posts look very different and it is confusing,
sorry for the mess I have made of all the posts and relative answers.

To recap:

The pond where the fish died was abandoned for months. The first week
I didn't even know there were fish in it. All the pumps but one were
disconnected and the one that was working did so poorly.

After reading all your kind answers, I believe the fish died of
poisoning from gasses released by the pond's bottom. The chlorine
level (my first suspicion) was fine, our water is very low in chlorine
(actually, no chlorine was found in the test) and I have had it tested
both before and after the accident. Due to the shabby constructions
and many leaks, the previous owners had a hose refilling the pond
every night with the irrigation system. That had gone on for months
without harming the fish.

As I said, not knowing better we went out and bought 12 baby kois, we
felt we had to replenish the once that died. Some are growing and some
are still very little. When they'll grow too big, my neighbor will
take them as she converted her swimming pool into a very large pond
(pool is half size Olympic). So I am not worried about them growing to
monster sizes.

I am still curious about the standards used to define populations of
koi Vs. pond capacity: My kois are very happy and at their present
size they hardly seem to populate the pond, even if they grow to the
sizes of their predecessors I don't see a problem.

I know have 3 pumps with biological filters and a supply of water
conditioners good enough to treat a small lake, plus I have made a
waterfall going from a pump into a large (very large) pot that was a
water garden once, there I put a skimmer pump to leave the residues at
the bottom and pump the water back into the pond. It looks pretty and
according to articles I read, it should act as a potent bio-filter.

Basically, I turned this pond into a very large aquarium. And, no
matter what, I am not getting rid of any of my Koi, unless they grow
big enough to beat me up.

My goal is not so much to have the prettiest pond around, but to host
the fish in a way they are happy and healthy.

One other note: I seem to remember that Goldfish, or Carps in general,
grow in relation to their habitat. Is this a myth? If it is not, than
how do you know what fish will grow large and what will not? It seems
to me that measuring Gallons=Fish# is not accurate because it doesn't
take into consideration how big the fish is. 10 baby koi get lost in
my pond, but if I were to get the monsters I have seen at my local
pond shop, one of them alone wouldn't be able to turn around.
Anyway, thank you for all your help