My fish are dying - please help !

Hi all,
I have a mature pond about 8 years old, 800 gallons, about 50 goldfish
ranging from tiny tiddlers to about 5" long, plus one really big ghost koi
about 18" long. I have all the usual filtration, waterfall and oxygenating
plants, UV light, water is nice and clean and clear and always has been. I
feed the fish about every couple of days - if they get too much they get
frisky.

Big problem - In the last week I've found 3 of the larger goldfish just
floating dead in the water. They have no signs of any disease and look
perfectly OK, just dead. This is very distressing as I'm spending every
moment checking the pond and expecting all my little friends to die. I put
some medifin in the pond a couple of days ago (I treat it twice a year
normally - Spring and Autumn), but I've had another death since then.

The only thing that's changed recently (don't know if it could possibly be
the reason) - I recently netted the pond because we were visited by a heron,
then I realised that the frogs and newts couldn't get in and out so I put a
length of wood into the water from the side. the wood is sapele (sp?) and
is definitely untreated, but I know some hardwoods have funny substances in
them - do you think this might have poisoned the water ?? What shall I do ?
Please get back to me soon - I'm desperate to save them.
Thanks, Carole