Hi Fishlady,
I don't have the results to hand right now as I'm at work but will post them later.
The pond I have is 4.2 meters by 3 meters and is 80 cm deep.
I'm running an Oase Aquamax Eco 12000 Pond Pump which I believe should be easily enough to oxygenate the water.
Gradually, the fish have started lying on their sides, floating around the top and eventually die.
I've lost 12 goldfish so far and have around 15 left (including 6 Koi).
Out of all the fish that have died, only one had any marks on it and that was that it's fins looked as though they had been nibbled on, no other fish have had that.
I know my current filter isn't going to be good enough once spring kicks in but I do have a much better filter on order so that will go in asap.
The garden centre who looked over one of my dead fish commented that it had a very firm stomach.
currently, I have no fish who look like they're unwell, just the one who is slashing at the top before swimming off again quite quickly.
What I've read so far has lead me to believe there is a parasite and I'm desperately hoping it's not flukes as I've also read it's very hard to do much about at this time of year due to the cold temperatures outside.
Water results to follow a bit later
Thanks
Lee