Vader's dead -maybe pond is sick!

I had my first death today :-(

I tested the water only yesterday and I didn't find anything wrong, however
Judging by other post's (do dead fish float) its possible Vader died a
couple of days ago but has only just floated to the surface.

My pond turned Green about Two weeks ago (I expected this and intended to
just sit still until the Green vanished naturally -I've had Barley straw in
their for about Three weeks but I know it takes a month to start working)

Before the water turned Green my fish were really getting confident -they
were swimming in and out of the plants in the shallow end and always came up
to get their food (as long as I hid) and they were all happy as Larry.

As soon as it turned Green -I felt a huge depression come over the pond and
the fish stopped coming to the surface -I've only seen Two of them in a
week! -I had a feeling they were dying even though the tests say everything
is fine.

Do you think that because the water is Green they can't see the food and are
starving to death? (I put food in their every day and it is disappearing but
maybe one particular fish stays near the surface and eats it all and the
other fish never know it was there -I tried staying to see who's eating it
but no-one will come up while I'm there.

The other day I noticed a huge underwater disturbance and stared at it for
ages trying to work out what was causing it -its hundreds of air bubbles
they seem to be causing a swell of some kind under the surface.When I first
put my fish in, everytime I went near them they hid under the pump and they
looked all squashed and uncomfortable so after speaking to the guy at the
pond shop he got about 20 oxygenators tied then together and when I got home
I weighted them with a rock and put them in the centre of the pond for them
to hid in -they loved it as soon it as soon as they saw it.
Now of course it has grown and seems to be creating lots of oxygen -is it
possible to have too much O2 in a pond?

My pond is 1000 litres and has a waterfall at one end, 6 small clumps of
oxygenators scattered around the sides and on shelves, the huge clump in the
middle, a large waterlilly with about 12 pads at the opposite end from the
waterfall, 7 marginals on the shelves and 6 floating plants which are
multiplying faster than rabbits.

Can anyone spot something stupid that I've done? I wanted to never use
chemicals but I'm now considering using an Algaecide to clear the water just
so I can see if the rest of the fish are suffering at the bottom -its really
worrying not to be able to see them.

I never expected the water to get so Green -just a Green colour -I have a
filter which has two sponges inside it which I expected to filter the Algae
out.

Any ideas please? (I know the Algaecide will cause a lot of dead Algae for
new Algae to eat and trap me in a vicious circle but do you think it's worth
doing it once just to make sure the fish aren't suffering?)

The dead fish looked in perfect condition by the way -no signs of sickness
maybe the Green water has worried it to death because it could not see the
sun anymore?

Help please,
Jay