Thank you all for the welcome.
The pond is 3M x 3M x 1M , 9000litres . Filter is a Oase Proficlear compcct with a 72w UV lamp, all water parameters are ok, Ph 7,5, Amonia 0.2, nitrite 0.25, phosphate 0. 8 air stones
I have approx 15 koi (4 are in 10lb range) 4 shrunken, 3 goldfish
Double pond size in August and noticed 2or 3 with red marks. Treated all pond and eventually transferred to 3x2x1metre pool whilst did work on new pond extension. Treated pool with Melafix for 7 days.
All put into new pond , with same water.
3 chagoi were added in July but were in isolation for 3 weeks and showed no sign of disease.