Beginner luck ?

Hi all,

This is my second post here, and is connected to my "salt in pond"
post I just submitted.

Background: I've had a 400 gallon pond stocked with Koi and goldfish
for over a year, and everything has gone fantastic. Good water
quality, happy, tame fish. I guess it was beginners' luck . . .

Then, two weeks ago, one of my koi (a solid pale gold that I really
liked) died from mouth rot (as diagnosed by my local pond supply
place). A check of the water revealed that pH had climbed from 7.1 to
8.9 in a couple of months (the last time I had checked), which totally
blew me away.

Prior to the fish's death, there was a behavioral change. The fish had
all become very skittish, and no longer tame. The pond place said
there was possibly a bird-predator, and that, combined with the pH
increase, may have stressed the fish and increased susceptibility to
the disease.

The pond place told me to use this malachite green product and change
the water over the course of the 3 daily applications. I followed the
instructions to the letter, and retested the pH, which was now back
down to 7.0. I also added a tiny bit of salt (see my other post
regarding why I am worried about using salt). My friends kidded me
when I told them what I was doing . . . they said I'd end up with
Ceviche.

Anyhoo . . . My reason for posting is this: The fish seemed to respond
well to the treatment . . . they became more active and behaved
better. Then a week after the treatment, disaster struck and a second
koi died!

I am now at a loss as to what to do: Was the second death due to
merely the fact the animal was already too far gone and was doomed to
death anyways despite the treatment --- and therefore I need not worry
about the remaining fish? Or did it die because the treatment was
ineffectual at curing the problem, and are all my other fish at risk
--- and therefore I better do something . . . but what? If malachite
green didn't work, what do I do? Or am I over-reacting?

I presume that the disease was introduced with an infected fish that I
unknowingly introduced into the pond. If this is the case, how do I
treat incoming fish before putting them in the pond? Do I put a few
drops of Malachite Green product into the bag with water and fish in
it? If so, how many drops and how long do I delay introducing the fish
to the pond? I don't have any other water reservoirs to put new fish
into , so the only approach I can think of is keeping the fish in the
bag a couple of days?

I hope the experts in this forum can help!

Thanks!

D