Hi Carole,

I have all the usual filtration, waterfall and
oxygenating plants, UV light

No air stones? Plants will not be enough.

I feed the fish about every couple of days - if they
get too much they get frisky.

That Ghost will be eating the little ones if you don't feed it more
often! I had 4 Ghosties of that size once ... When they were hungry
they'd try and eat the paving slabs around the edge of the pond!

Big problem - In the last week I've found 3 of the larger goldfish
just floating dead in the water.

Are the other fish gasping at the surface? I'd be running a KA25 with
that level of stock.

I put some medifin in the pond a couple of days ago
(I treat it twice a year normally - Spring and Autumn),

Reasonable enough, but if the fish don't look distressed or diseased, I
wouldn't bother. I used to treat with Medifin like you are, but I haven't
bothered for a few years as it simply wasn't necessary.

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

The old "fishing line tied around canes" trick works best!

hardwoods have funny substances in them - do you think this might have
poisoned the water ??

I doubt it. Given the amount of crap that ends up in my pond (leaves,
branches, mice, hedgehogs, dead frogs ...) and that I've not had problems
would suggest not.

What shall I do ?

Buy an air pump, some tubing, a few airstones, and see what happens.
Certainly won't cause any harm.

Also, talk to your local aquatic centre people and ask if they've had
similar problems reported from customers.

Cheers,

Al.